Debugging

If you invoke Perl with a -d switch, your script will be run under a debugging monitor. It will halt before the first executable statement and ask you for a command, such as:

h: Prints out a help message.
T: Stack trace.
s: Single step. Executes until it reaches the beginning of another statement.
n: Next. Executes over subroutine calls, until it reaches the beginning of the next statement.
f: Finish. Executes statements until it has finished the current subroutine.
c: Continue. Executes until the next breakpoint is reached.
c line: Continue to the specified line. Inserts a one-time-only breakpoint at the specified line.
<CR>: Repeat last n or s.
l min+incr: List incr+1 lines starting at min. If min is omitted, starts where last listing left off. If incr is omitted, previous value of incr is used.
l min-max: List lines in the indicated range.
l line: List just the indicated line.
l: List next window.
-: List previous window.
w line: List window around line.
l subname: List subroutine. If it's a long subroutine it just lists the beginning. Use "l" to list more.
/pattern/: Regular expression search forward for pattern; the final / is optional.
?pattern?: Regular expression search backward for pattern; the final ? is optional.
L: List lines that have breakpoints or actions.
S: Lists the names of all subroutines.
t: Toggle trace mode on or off.
b line condition: Set a breakpoint. If line is omitted, sets a breakpoint on the line that is about to be executed. If a condition is specified, it is evaluated each time the statement is reached and a breakpoint is taken only if the condition is true. Breakpoints may only be set on lines that begin an executable statement.
b subname condition: Set breakpoint at first executable line of subroutine.
<dt>d line; Delete breakpoint. If line is omitted, deletes the breakpoint on the line that is about to be executed.
D: Delete all breakpoints.
a line command: Set an action for line. A multi-line command may be entered by backslashing the newlines.
A: Delete all line actions.
< command: Set an action to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines.
> command: Set an action 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.
V package: List all variables in package. Default is main package.
! number: Redo a debugging command. If number is omitted, redoes the previous command.
! -number: Redo the command that was that many commands ago.
H -number: Display last n commands. Only commands longer than one character are listed. If number is omitted, lists them all.
q or ^D: Quit.
command: Execute command as a perl statement. A missing semicolon will be supplied.
p expr: Same as "print DB'OUT expr". The DB'OUT filehandle is opened to /dev/tty, regardless of where STDOUT may be redirected to.