How do I move the instruction point in the Perl debugger?

Question

I would like to be able to (reasonably) arbitrarily set my execution point in the Perl debugger. E.g., moving immediately prior to an if from the body of the if and setting a variable.

Rummaging around the perldebug(and perldebguts and the perl debugger POD) page suggests that this kind of functionality is either not supported or not documented.

Answer 1

A cumbersome workaround would be to add labels and conditional goto statements throughout your code. But depending on how badly you want to emulate this feature, it might be worth it.

POINT1: $GOTO="";      # $GOTO is our fake variable that we only set from the debugger
($a,$b,$c)=(1,2,3);
POINT2: $GOTO="";
if ($a < $b) {
    goto $GOTO if $GOTO;
    if ($a > $c) {
        goto $GOTO if $GOTO;
        print "foo\n";
    } else {
        goto $GOTO if $GOTO;
        print "bar\n";
    }
    goto $GOTO if $GOTO;
 } else {
    goto $GOTO if $GOTO;
    print "nothing\n";
    goto $GOTO if $GOTO;
 }

Sample debugging session:

$ perl -d debuggoto.pl

Loading DB routines from perl5db.pl version 1.28
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(debuggoto.pl:1): POINT1: $GOTO="";      # $GOTO is our fake variable that we only set from the debugger
  DB<1> n
main::(debuggoto.pl:2): ($a,$b,$c)=(1,2,3);
  DB<1>
main::(debuggoto.pl:3): POINT2: $GOTO="";
  DB<1>
main::(debuggoto.pl:4): if ($a < $b) {
  DB<1>
main::(debuggoto.pl:5):    goto $GOTO if $GOTO;
  DB<1>
main::(debuggoto.pl:6):    if ($a > $c) {
  DB<1>
main::(debuggoto.pl:10):               goto $GOTO if $GOTO;
  DB<1>
main::(debuggoto.pl:11):               print "bar\n";
  DB<1>
bar
main::(debuggoto.pl:13):           goto $GOTO if $GOTO;
  DB<1> $GOTO="POINT2"

  DB<2> n
main::(debuggoto.pl:3): POINT2: $GOTO="";
  DB<2> $c=0

  DB<3> n
main::(debuggoto.pl:4): if ($a < $b) {
  DB<3>
main::(debuggoto.pl:5):    goto $GOTO if $GOTO;
  DB<3>
main::(debuggoto.pl:6):    if ($a > $c) {
  DB<3>
main::(debuggoto.pl:7):        goto $GOTO if $GOTO;
  DB<3>
main::(debuggoto.pl:8):        print "foo\n";
  DB<3>
foo
main::(debuggoto.pl:13):           goto $GOTO if $GOTO;
  DB<3>
Debugged program terminated.  Use q to quit or R to restart,
  use o inhibit_exit to avoid stopping after program termination,
  h q, h R or h o to get additional info.
  DB<3>
Use `q' to quit or `R' to restart.  `h q' for details.
  DB<3>

I wonder if it would be possible to build a debugger that uses this idea.