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PERLDEBUG(1pm)         Perl Programmers Reference Guide         PERLDEBUG(1pm)


       perldebug - Perl debugging


       First of all, have you tried using "use strict;" and "use warnings;"?

       If you're new to the Perl debugger, you may prefer to read perldebtut,
       which is a tutorial introduction to the debugger.

       If you're looking for the nitty gritty details of how the debugger is
       implemented, you may prefer to read perldebguts.

       For in-depth technical usage details, see, the documentation
       of the debugger itself.

The Perl Debugger

       If you invoke Perl with the -d switch, your script runs under the Perl
       source debugger.  This works like an interactive Perl environment,
       prompting for debugger commands that let you examine source code, set
       breakpoints, get stack backtraces, change the values of variables, etc.
       This is so convenient that you often fire up the debugger all by itself
       just to test out Perl constructs interactively to see what they do.
       For example:

           $ perl -d -e 42

       In Perl, the debugger is not a separate program the way it usually is
       in the typical compiled environment.  Instead, the -d flag tells the
       compiler to insert source information into the parse trees it's about
       to hand off to the interpreter.  That means your code must first
       compile correctly for the debugger to work on it.  Then when the
       interpreter starts up, it preloads a special Perl library file
       containing the debugger.

       The program will halt right before the first run-time executable
       statement (but see below regarding compile-time statements) and ask you
       to enter a debugger command.  Contrary to popular expectations,
       whenever the debugger halts and shows you a line of code, it always
       displays the line it's about to execute, rather than the one it has
       just executed.

       Any command not recognized by the debugger is directly executed
       ("eval"'d) as Perl code in the current package.  (The debugger uses the
       DB package for keeping its own state information.)

       Note that the said "eval" is bound by an implicit scope. As a result
       any newly introduced lexical variable or any modified capture buffer
       content is lost after the eval. The debugger is a nice environment to
       learn Perl, but if you interactively experiment using material which
       should be in the same scope, stuff it in one line.

       For any text entered at the debugger prompt, leading and trailing
       whitespace is first stripped before further processing.  If a debugger
       command coincides with some function in your own program, merely
       precede the function with something that doesn't look like a debugger
       command, such as a leading ";" or perhaps a "+", or by wrapping it with
       parentheses or braces.

   Calling the Debugger
       There are several ways to call the debugger:

       perl -d program_name
           On the given program identified by "program_name".

       perl -d -e 0
           Interactively supply an arbitrary "expression" using "-e".

       perl -d:ptkdb program_name
           Debug a given program via the "Devel::ptkdb" GUI.

       perl -dt threaded_program_name
           Debug a given program using threads (experimental).

   Debugger Commands
       The interactive debugger understands the following commands:

       h           Prints out a summary help message

       h [command] Prints out a help message for the given debugger command.

       h h         The special argument of "h h" produces the entire help
                   page, which is quite long.

                   If the output of the "h h" command (or any command, for
                   that matter) scrolls past your screen, precede the command
                   with a leading pipe symbol so that it's run through your
                   pager, as in

                       DB> |h h

                   You may change the pager which is used via "o pager=..."

       p expr      Same as "print {$DB::OUT} expr" in the current package.  In
                   particular, because this is just Perl's own "print"
                   function, this means that nested data structures and
                   objects are not dumped, unlike with the "x" command.

                   The "DB::OUT" filehandle is opened to /dev/tty, regardless
                   of where STDOUT may be redirected to.

       x [maxdepth] expr
                   Evaluates its expression in list context and dumps out the
                   result in a pretty-printed fashion.  Nested data structures
                   are printed out recursively, unlike the real "print"
                   function in Perl.  When dumping hashes, you'll probably
                   prefer 'x \%h' rather than 'x %h'.  See Dumpvalue if you'd
                   like to do this yourself.

                   The output format is governed by multiple options described
                   under "Configurable Options".

                   If the "maxdepth" is included, it must be a numeral N; the
                   value is dumped only N levels deep, as if the "dumpDepth"
                   option had been temporarily set to N.

       V [pkg [vars]]
                   Display all (or some) variables in package (defaulting to
                   "main") using a data pretty-printer (hashes show their keys
                   and values so you see what's what, control characters are
                   made printable, etc.).  Make sure you don't put the type
                   specifier (like "$") there, just the symbol names, like

                       V DB filename line

                   Use "~pattern" and "!pattern" for positive and negative

                   This is similar to calling the "x" command on each
                   applicable var.

       X [vars]    Same as "V currentpackage [vars]".

       y [level [vars]]
                   Display all (or some) lexical variables (mnemonic: "mY"
                   variables) in the current scope or level scopes higher.
                   You can limit the variables that you see with vars which
                   works exactly as it does for the "V" and "X" commands.
                   Requires the "PadWalker" module version 0.08 or higher;
                   will warn if this isn't installed.  Output is pretty-
                   printed in the same style as for "V" and the format is
                   controlled by the same options.

       T           Produce a stack backtrace.  See below for details on its

       s [expr]    Single step.  Executes until the beginning of another
                   statement, descending into subroutine calls.  If an
                   expression is supplied that includes function calls, it too
                   will be single-stepped.

       n [expr]    Next.  Executes over subroutine calls, until the beginning
                   of the next statement.  If an expression is supplied that
                   includes function calls, those functions will be executed
                   with stops before each statement.

       r           Continue until the return from the current subroutine.
                   Dump the return value if the "PrintRet" option is set

       <CR>        Repeat last "n" or "s" command.

       c [line|sub]
                   Continue, optionally inserting a one-time-only breakpoint
                   at the specified line or subroutine.

       l           List next window of lines.

       l min+incr  List "incr+1" lines starting at "min".

       l min-max   List lines "min" through "max".  "l -" is synonymous to

       l line      List a single line.

       l subname   List first window of lines from subroutine.  subname may be
                   a variable that contains a code reference.

       -           List previous window of lines.

       v [line]    View a few lines of code around the current line.

       .           Return the internal debugger pointer to the line last
                   executed, and print out that line.

       f filename  Switch to viewing a different file or "eval" statement.  If
                   filename is not a full pathname found in the values of
                   %INC, it is considered a regex.

                   "eval"ed strings (when accessible) are considered to be
                   filenames: "f (eval 7)" and "f eval 7\b" access the body of
                   the 7th "eval"ed string (in the order of execution).  The
                   bodies of the currently executed "eval" and of "eval"ed
                   strings that define subroutines are saved and thus

       /pattern/   Search forwards for pattern (a Perl regex); final / is
                   optional.  The search is case-insensitive by default.

       ?pattern?   Search backwards for pattern; final ? is optional.  The
                   search is case-insensitive by default.

       L [abw]     List (default all) actions, breakpoints and watch

       S [[!]regex]
                   List subroutine names [not] matching the regex.

       t [n]       Toggle trace mode (see also the "AutoTrace" option).
                   Optional argument is the maximum number of levels to trace
                   below the current one; anything deeper than that will be

       t [n] expr  Trace through execution of "expr".  Optional first argument
                   is the maximum number of levels to trace below the current
                   one; anything deeper than that will be silent.  See "Frame
                   Listing Output Examples" in perldebguts for examples.

       b           Sets breakpoint on current line

       b [line] [condition]
                   Set a breakpoint before the given line.  If a condition is
                   specified, it's evaluated each time the statement is
                   reached: a breakpoint is taken only if the condition is
                   true.  Breakpoints may only be set on lines that begin an
                   executable statement.  Conditions don't use "if":

                       b 237 $x > 30
                       b 237 ++$count237 < 11
                       b 33 /pattern/i

                   If the line number is ".", sets a breakpoint on the current

                       b . $n > 100

       b [file]:[line] [condition]
                   Set a breakpoint before the given line in a (possibly
                   different) file.  If a condition is specified, it's
                   evaluated each time the statement is reached: a breakpoint
                   is taken only if the condition is true.  Breakpoints may
                   only be set on lines that begin an executable statement.
                   Conditions don't use "if":

                       b lib/ $x > 30
                       b /usr/lib/perl5/site_perl/ ++$count100 < 11

       b subname [condition]
                   Set a breakpoint before the first line of the named
                   subroutine.  subname may be a variable containing a code
                   reference (in this case condition is not supported).

       b postpone subname [condition]
                   Set a breakpoint at first line of subroutine after it is

       b load filename
                   Set a breakpoint before the first executed line of the
                   filename, which should be a full pathname found amongst the
                   %INC values.

       b compile subname
                   Sets a breakpoint before the first statement executed after
                   the specified subroutine is compiled.

       B line      Delete a breakpoint from the specified line.

       B *         Delete all installed breakpoints.

       disable [file]:[line]
                   Disable the breakpoint so it won't stop the execution of
                   the program.  Breakpoints are enabled by default and can be
                   re-enabled using the "enable" command.

       disable [line]
                   Disable the breakpoint so it won't stop the execution of
                   the program.  Breakpoints are enabled by default and can be
                   re-enabled using the "enable" command.

                   This is done for a breakpoint in the current file.

       enable [file]:[line]
                   Enable the breakpoint so it will stop the execution of the

       enable [line]
                   Enable the breakpoint so it will stop the execution of the

                   This is done for a breakpoint in the current file.

       a [line] command
                   Set an action to be done before the line is executed.  If
                   line is omitted, set an action on the line about to be
                   executed.  The sequence of steps taken by the debugger is

                     1. check for a breakpoint at this line
                     2. print the line if necessary (tracing)
                     3. do any actions associated with that line
                     4. prompt user if at a breakpoint or in single-step
                     5. evaluate line

                   For example, this will print out $foo every time line 53 is

                       a 53 print "DB FOUND $foo\n"

       A line      Delete an action from the specified line.

       A *         Delete all installed actions.

       w expr      Add a global watch-expression. Whenever a watched global
                   changes the debugger will stop and display the old and new

       W expr      Delete watch-expression

       W *         Delete all watch-expressions.

       o           Display all options.

       o booloption ...
                   Set each listed Boolean option to the value 1.

       o anyoption? ...
                   Print out the value of one or more options.

       o option=value ...
                   Set the value of one or more options.  If the value has
                   internal whitespace, it should be quoted.  For example, you
                   could set "o pager="less -MQeicsNfr"" to call less with
                   those specific options.  You may use either single or
                   double quotes, but if you do, you must escape any embedded
                   instances of same sort of quote you began with, as well as
                   any escaping any escapes that immediately precede that
                   quote but which are not meant to escape the quote itself.
                   In other words, you follow single-quoting rules
                   irrespective of the quote; eg: "o option='this isn\'t bad'"
                   or "o option="She said, \"Isn't it?\""".

                   For historical reasons, the "=value" is optional, but
                   defaults to 1 only where it is safe to do so--that is,
                   mostly for Boolean options.  It is always better to assign
                   a specific value using "=".  The "option" can be
                   abbreviated, but for clarity probably should not be.
                   Several options can be set together.  See "Configurable
                   Options" for a list of these.

       < ?         List out all pre-prompt Perl command actions.

       < [ command ]
                   Set an action (Perl command) to happen before every
                   debugger prompt.  A multi-line command may be entered by
                   backslashing the newlines.

       < *         Delete all pre-prompt Perl command actions.

       << command  Add an action (Perl command) to happen before every
                   debugger prompt.  A multi-line command may be entered by
                   backwhacking the newlines.

       > ?         List out post-prompt Perl command actions.

       > command   Set an action (Perl command) to happen after the prompt
                   when you've just given a command to return to executing the
                   script.  A multi-line command may be entered by
                   backslashing the newlines (we bet you couldn't have guessed
                   this by now).

       > *         Delete all post-prompt Perl command actions.

       >> command  Adds an action (Perl command) to happen after the prompt
                   when you've just given a command to return to executing the
                   script.  A multi-line command may be entered by
                   backslashing the newlines.

       { ?         List out pre-prompt debugger commands.

       { [ command ]
                   Set an action (debugger command) to happen before every
                   debugger prompt.  A multi-line command may be entered in
                   the customary fashion.

                   Because this command is in some senses new, a warning is
                   issued if you appear to have accidentally entered a block
                   instead.  If that's what you mean to do, write it as with
                   ";{ ... }" or even "do { ... }".

       { *         Delete all pre-prompt debugger commands.

       {{ command  Add an action (debugger command) to happen before every
                   debugger prompt.  A multi-line command may be entered, if
                   you can guess how: see above.

       ! number    Redo a previous command (defaults to the previous command).

       ! -number   Redo number'th previous command.

       ! pattern   Redo last command that started with pattern.  See "o
                   recallCommand", too.

       !! cmd      Run cmd in a subprocess (reads from DB::IN, writes to
                   DB::OUT) See "o shellBang", also.  Note that the user's
                   current shell (well, their $ENV{SHELL} variable) will be
                   used, which can interfere with proper interpretation of
                   exit status or signal and coredump information.

       source file Read and execute debugger commands from file.  file may
                   itself contain "source" commands.

       H -number   Display last n commands.  Only commands longer than one
                   character are listed.  If number is omitted, list them all.

       q or ^D     Quit.  ("quit" doesn't work for this, unless you've made an
                   alias) This is the only supported way to exit the debugger,
                   though typing "exit" twice might work.

                   Set the "inhibit_exit" option to 0 if you want to be able
                   to step off the end the script.  You may also need to set
                   $finished to 0 if you want to step through global

       R           Restart the debugger by "exec()"ing a new session.  We try
                   to maintain your history across this, but internal settings
                   and command-line options may be lost.

                   The following setting are currently preserved: history,
                   breakpoints, actions, debugger options, and the Perl
                   command-line options -w, -I, and -e.

       |dbcmd      Run the debugger command, piping DB::OUT into your current

       ||dbcmd     Same as "|dbcmd" but DB::OUT is temporarily "select"ed as

       = [alias value]
                   Define a command alias, like

                       = quit q

                   or list current aliases.

       command     Execute command as a Perl statement.  A trailing semicolon
                   will be supplied.  If the Perl statement would otherwise be
                   confused for a Perl debugger, use a leading semicolon, too.

       m expr      List which methods may be called on the result of the
                   evaluated expression.  The expression may evaluated to a
                   reference to a blessed object, or to a package name.

       M           Display all loaded modules and their versions.

       man [manpage]
                   Despite its name, this calls your system's default
                   documentation viewer on the given page, or on the viewer
                   itself if manpage is omitted.  If that viewer is man, the
                   current "Config" information is used to invoke man using
                   the proper MANPATH or -M manpath option.  Failed lookups of
                   the form "XXX" that match known manpages of the form
                   perlXXX will be retried.  This lets you type "man debug" or
                   "man op" from the debugger.

                   On systems traditionally bereft of a usable man command,
                   the debugger invokes perldoc.  Occasionally this
                   determination is incorrect due to recalcitrant vendors or
                   rather more felicitously, to enterprising users.  If you
                   fall into either category, just manually set the
                   $DB::doccmd variable to whatever viewer to view the Perl
                   documentation on your system.  This may be set in an rc
                   file, or through direct assignment.  We're still waiting
                   for a working example of something along the lines of:

                       $DB::doccmd = 'netscape -remote';

   Configurable Options
       The debugger has numerous options settable using the "o" command,
       either interactively or from the environment or an rc file. The file is
       named ./.perldb or ~/.perldb under Unix with /dev/tty, perldb.ini

       "recallCommand", "ShellBang"
                   The characters used to recall a command or spawn a shell.
                   By default, both are set to "!", which is unfortunate.

       "pager"     Program to use for output of pager-piped commands (those
                   beginning with a "|" character.)  By default, $ENV{PAGER}
                   will be used.  Because the debugger uses your current
                   terminal characteristics for bold and underlining, if the
                   chosen pager does not pass escape sequences through
                   unchanged, the output of some debugger commands will not be
                   readable when sent through the pager.

       "tkRunning" Run Tk while prompting (with ReadLine).

       "signalLevel", "warnLevel", "dieLevel"
                   Level of verbosity.  By default, the debugger leaves your
                   exceptions and warnings alone, because altering them can
                   break correctly running programs.  It will attempt to print
                   a message when uncaught INT, BUS, or SEGV signals arrive.
                   (But see the mention of signals in "BUGS" below.)

                   To disable this default safe mode, set these values to
                   something higher than 0.  At a level of 1, you get
                   backtraces upon receiving any kind of warning (this is
                   often annoying) or exception (this is often valuable).
                   Unfortunately, the debugger cannot discern fatal exceptions
                   from non-fatal ones.  If "dieLevel" is even 1, then your
                   non-fatal exceptions are also traced and unceremoniously
                   altered if they came from "eval'ed" strings or from any
                   kind of "eval" within modules you're attempting to load.
                   If "dieLevel" is 2, the debugger doesn't care where they
                   came from:  It usurps your exception handler and prints out
                   a trace, then modifies all exceptions with its own
                   embellishments.  This may perhaps be useful for some
                   tracing purposes, but tends to hopelessly destroy any
                   program that takes its exception handling seriously.

       "AutoTrace" Trace mode (similar to "t" command, but can be put into

       "LineInfo"  File or pipe to print line number info to.  If it is a pipe
                   (say, "|visual_perl_db"), then a short message is used.
                   This is the mechanism used to interact with a slave editor
                   or visual debugger, such as the special "vi" or "emacs"
                   hooks, or the "ddd" graphical debugger.

                   If 0, allows stepping off the end of the script.

       "PrintRet"  Print return value after "r" command if set (default).

       "ornaments" Affects screen appearance of the command line (see
                   Term::ReadLine).  There is currently no way to disable
                   these, which can render some output illegible on some
                   displays, or with some pagers.  This is considered a bug.

       "frame"     Affects the printing of messages upon entry and exit from
                   subroutines.  If "frame & 2" is false, messages are printed
                   on entry only. (Printing on exit might be useful if
                   interspersed with other messages.)

                   If "frame & 4", arguments to functions are printed, plus
                   context and caller info.  If "frame & 8", overloaded
                   "stringify" and "tie"d "FETCH" is enabled on the printed
                   arguments.  If "frame & 16", the return value from the
                   subroutine is printed.

                   The length at which the argument list is truncated is
                   governed by the next option:

                   Length to truncate the argument list when the "frame"
                   option's bit 4 is set.

                   Change the size of code list window (default is 10 lines).

       The following options affect what happens with "V", "X", and "x"

       "arrayDepth", "hashDepth"
                   Print only first N elements ('' for all).

       "dumpDepth" Limit recursion depth to N levels when dumping structures.
                   Negative values are interpreted as infinity.  Default:

       "compactDump", "veryCompact"
                   Change the style of array and hash output.  If
                   "compactDump", short array may be printed on one line.

       "globPrint" Whether to print contents of globs.

                   Dump arrays holding debugged files.

                   Dump symbol tables of packages.

                   Dump contents of "reused" addresses.

       "quote", "HighBit", "undefPrint"
                   Change the style of string dump.  The default value for
                   "quote" is "auto"; one can enable double-quotish or single-
                   quotish format by setting it to """ or "'", respectively.
                   By default, characters with their high bit set are printed

       "UsageOnly" Rudimentary per-package memory usage dump.  Calculates
                   total size of strings found in variables in the package.
                   This does not include lexicals in a module's file scope, or
                   lost in closures.

       "HistFile"  The path of the file from which the history (assuming a
                   usable Term::ReadLine backend) will be read on the
                   debugger's startup, and to which it will be saved on
                   shutdown (for persistence across sessions). Similar in
                   concept to Bash's ".bash_history" file.

       "HistSize"  The count of the saved lines in the history (assuming
                   "HistFile" above).

       After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS}
       environment variable and parses this as the remainder of a "O ..."
       line as one might enter at the debugger prompt.  You may place the
       initialization options "TTY", "noTTY", "ReadLine", and "NonStop" there.

       If your rc file contains:

         parse_options("NonStop=1 LineInfo=db.out AutoTrace");

       then your script will run without human intervention, putting trace
       information into the file db.out.  (If you interrupt it, you'd better
       reset "LineInfo" to /dev/tty if you expect to see anything.)

       "TTY"       The TTY to use for debugging I/O.

       "noTTY"     If set, the debugger goes into "NonStop" mode and will not
                   connect to a TTY.  If interrupted (or if control goes to
                   the debugger via explicit setting of $DB::signal or
                   $DB::single from the Perl script), it connects to a TTY
                   specified in the "TTY" option at startup, or to a tty found
                   at runtime using the "Term::Rendezvous" module of your

                   This module should implement a method named "new" that
                   returns an object with two methods: "IN" and "OUT".  These
                   should return filehandles to use for debugging input and
                   output correspondingly.  The "new" method should inspect an
                   argument containing the value of $ENV{PERLDB_NOTTY} at
                   startup, or "$ENV{HOME}/.perldbtty$$" otherwise.  This file
                   is not inspected for proper ownership, so security hazards
                   are theoretically possible.

       "ReadLine"  If false, readline support in the debugger is disabled in
                   order to debug applications that themselves use ReadLine.

       "NonStop"   If set, the debugger goes into non-interactive mode until
                   interrupted, or programmatically by setting $DB::signal or

       Here's an example of using the $ENV{PERLDB_OPTS} variable:

           $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram

       That will run the script myprogram without human intervention, printing
       out the call tree with entry and exit points.  Note that "NonStop=1
       frame=2" is equivalent to "N f=2", and that originally, options could
       be uniquely abbreviated by the first letter (modulo the "Dump*"
       options).  It is nevertheless recommended that you always spell them
       out in full for legibility and future compatibility.

       Other examples include

           $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram

       which runs script non-interactively, printing info on each entry into a
       subroutine and each executed line into the file named listing.  (If you
       interrupt it, you would better reset "LineInfo" to something

       Other examples include (using standard shell syntax to show environment
       variable settings):

         $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
             perl -d myprogram )

       which may be useful for debugging a program that uses "Term::ReadLine"
       itself.  Do not forget to detach your shell from the TTY in the window
       that corresponds to /dev/ttyXX, say, by issuing a command like

         $ sleep 1000000

       See "Debugger Internals" in perldebguts for details.

   Debugger Input/Output
       Prompt  The debugger prompt is something like


               or even


               where that number is the command number, and which you'd use to
               access with the built-in csh-like history mechanism.  For
               example, "!17" would repeat command number 17.  The depth of
               the angle brackets indicates the nesting depth of the debugger.
               You could get more than one set of brackets, for example, if
               you'd already at a breakpoint and then printed the result of a
               function call that itself has a breakpoint, or you step into an
               expression via "s/n/t expression" command.

       Multiline commands
               If you want to enter a multi-line command, such as a subroutine
               definition with several statements or a format, escape the
               newline that would normally end the debugger command with a
               backslash.  Here's an example:

                     DB<1> for (1..4) {         \
                     cont:     print "ok\n";   \
                     cont: }

               Note that this business of escaping a newline is specific to
               interactive commands typed into the debugger.

       Stack backtrace
               Here's an example of what a stack backtrace via "T" command
               might look like:

                $ = main::infested called from file '' line 10
                @ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea'
                                                                         line 7
                $ = main::pests('bactrian', 4) called from file 'camel_flea'
                                                                         line 4

               The left-hand character up there indicates the context in which
               the function was called, with "$" and "@" meaning scalar or
               list contexts respectively, and "." meaning void context (which
               is actually a sort of scalar context).  The display above says
               that you were in the function "main::infested" when you ran the
               stack dump, and that it was called in scalar context from line
               10 of the file, but without any arguments at all,
               meaning it was called as &infested.  The next stack frame shows
               that the function "Ambulation::legs" was called in list context
               from the camel_flea file with four arguments.  The last stack
               frame shows that "main::pests" was called in scalar context,
               also from camel_flea, but from line 4.

               If you execute the "T" command from inside an active "use"
               statement, the backtrace will contain both a "require" frame
               and an "eval" frame.

       Line Listing Format
               This shows the sorts of output the "l" command can produce:

                  DB<<13>> l
                101:        @i{@i} = ();
                102:b       @isa{@i,$pack} = ()
                103             if(exists $i{$prevpack} || exists $isa{$pack});
                104     }
                106     next
                107==>      if(exists $isa{$pack});
                109:a   if ($extra-- > 0) {
                110:        %isa = ($pack,1);

               Breakable lines are marked with ":".  Lines with breakpoints
               are marked by "b" and those with actions by "a".  The line
               that's about to be executed is marked by "==>".

               Please be aware that code in debugger listings may not look the
               same as your original source code.  Line directives and
               external source filters can alter the code before Perl sees it,
               causing code to move from its original positions or take on
               entirely different forms.

       Frame listing
               When the "frame" option is set, the debugger would print
               entered (and optionally exited) subroutines in different
               styles.  See perldebguts for incredibly long examples of these.

   Debugging Compile-Time Statements
       If you have compile-time executable statements (such as code within
       BEGIN, UNITCHECK and CHECK blocks or "use" statements), these will not
       be stopped by debugger, although "require"s and INIT blocks will, and
       compile-time statements can be traced with the "AutoTrace" option set
       in "PERLDB_OPTS").  From your own Perl code, however, you can transfer
       control back to the debugger using the following statement, which is
       harmless if the debugger is not running:

           $DB::single = 1;

       If you set $DB::single to 2, it's equivalent to having just typed the
       "n" command, whereas a value of 1 means the "s" command.  The
       $DB::trace  variable should be set to 1 to simulate having typed the
       "t" command.

       Another way to debug compile-time code is to start the debugger, set a
       breakpoint on the load of some module:

           DB<7> b load f:/perllib/lib/
         Will stop on load of 'f:/perllib/lib/'.

       and then restart the debugger using the "R" command (if possible).  One
       can use "b compile subname" for the same purpose.

   Debugger Customization
       The debugger probably contains enough configuration hooks that you
       won't ever have to modify it yourself.  You may change the behaviour of
       the debugger from within the debugger using its "o" command, from the
       command line via the "PERLDB_OPTS" environment variable, and from
       customization files.

       You can do some customization by setting up a .perldb file, which
       contains initialization code.  For instance, you could make aliases
       like these (the last one is one people expect to be there):

           $DB::alias{'len'}  = 's/^len(.*)/p length($1)/';
           $DB::alias{'stop'} = 's/^stop (at|in)/b/';
           $DB::alias{'ps'}   = 's/^ps\b/p scalar /';
           $DB::alias{'quit'} = 's/^quit(\s*)/exit/';

       You can change options from .perldb by using calls like this one;

           parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");

       The code is executed in the package "DB".  Note that .perldb is
       processed before processing "PERLDB_OPTS".  If .perldb defines the
       subroutine "afterinit", that function is called after debugger
       initialization ends.  .perldb may be contained in the current
       directory, or in the home directory.  Because this file is sourced in
       by Perl and may contain arbitrary commands, for security reasons, it
       must be owned by the superuser or the current user, and writable by no
       one but its owner.

       You can mock TTY input to debugger by adding arbitrary commands to
       @DB::typeahead. For example, your .perldb file might contain:

           sub afterinit { push @DB::typeahead, "b 4", "b 6"; }

       Which would attempt to set breakpoints on lines 4 and 6 immediately
       after debugger initialization. Note that @DB::typeahead is not a
       supported interface and is subject to change in future releases.

       If you want to modify the debugger, copy from the Perl
       library to another name and hack it to your heart's content.  You'll
       then want to set your "PERL5DB" environment variable to say something
       like this:

           BEGIN { require "" }

       As a last resort, you could also use "PERL5DB" to customize the
       debugger by directly setting internal variables or calling debugger

       Note that any variables and functions that are not documented in this
       document (or in perldebguts) are considered for internal use only, and
       as such are subject to change without notice.

   Readline Support / History in the Debugger
       As shipped, the only command-line history supplied is a simplistic one
       that checks for leading exclamation points.  However, if you install
       the Term::ReadKey and Term::ReadLine modules from CPAN (such as
       Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have full
       editing capabilities much like those GNU readline(3) provides.  Look
       for these in the modules/by-module/Term directory on CPAN.  These do
       not support normal vi command-line editing, however.

       A rudimentary command-line completion is also available, including
       lexical variables in the current scope if the "PadWalker" module is

       Without Readline support you may see the symbols "^[[A", "^[[C",
       "^[[B", "^[[D"", "^H", ... when using the arrow keys and/or the
       backspace key.

   Editor Support for Debugging
       If you have the GNU's version of emacs installed on your system, it can
       interact with the Perl debugger to provide an integrated software
       development environment reminiscent of its interactions with C

       Recent versions of Emacs come with a start file for making emacs act
       like a syntax-directed editor that understands (some of) Perl's syntax.
       See perlfaq3.

       Users of vi should also look into vim and gvim, the mousey and windy
       version, for coloring of Perl keywords.

       Note that only perl can truly parse Perl, so all such CASE tools fall
       somewhat short of the mark, especially if you don't program your Perl
       as a C programmer might.

   The Perl Profiler
       If you wish to supply an alternative debugger for Perl to run, invoke
       your script with a colon and a package argument given to the -d flag.
       Perl's alternative debuggers include a Perl profiler, Devel::NYTProf,
       which is available separately as a CPAN distribution.  To profile your
       Perl program in the file, just type:

           $ perl -d:NYTProf

       When the script terminates the profiler will create a database of the
       profile information that you can turn into reports using the profiler's
       tools. See <perlperf> for details.

Debugging Regular Expressions

       "use re 'debug'" enables you to see the gory details of how the Perl
       regular expression engine works. In order to understand this typically
       voluminous output, one must not only have some idea about how regular
       expression matching works in general, but also know how Perl's regular
       expressions are internally compiled into an automaton. These matters
       are explored in some detail in "Debugging Regular Expressions" in

Debugging Memory Usage

       Perl contains internal support for reporting its own memory usage, but
       this is a fairly advanced concept that requires some understanding of
       how memory allocation works.  See "Debugging Perl Memory Usage" in
       perldebguts for the details.


       You do have "use strict" and "use warnings" enabled, don't you?

       perldebtut(1), perldebguts(1), re(3), DB(3), Devel::NYTProf(3), 
       Dumpvalue(3), and perlrun(1).

       When debugging a script that uses #! and is thus normally found in
       $PATH, the -S option causes perl to search $PATH for it, so you don't
       have to type the path or "which $scriptname".

         $ perl -Sd


       You cannot get stack frame information or in any fashion debug
       functions that were not compiled by Perl, such as those from C or C++

       If you alter your @_ arguments in a subroutine (such as with "shift" or
       "pop"), the stack backtrace will not show the original values.

       The debugger does not currently work in conjunction with the -W
       command-line switch, because it itself is not free of warnings.

       If you're in a slow syscall (like "wait"ing, "accept"ing, or "read"ing
       from your keyboard or a socket) and haven't set up your own $SIG{INT}
       handler, then you won't be able to CTRL-C your way back to the
       debugger, because the debugger's own $SIG{INT} handler doesn't
       understand that it needs to raise an exception to longjmp(3) out of
       slow syscalls.

perl v5.34.0                      2020-10-24                    PERLDEBUG(1pm)

perl 5.34.0 - Generated Fri Feb 25 16:09:32 CST 2022
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