$ perl -d watchpoint-test.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::(watchpoint-test.pl:2):        for my $c (1..1000) {
  DB<1> w $onto{'PROBLEM'}{'mappings'}
  DB<2> c
Watchpoint 0:   $onto{'PROBLEM'}{'mappings'} changed:
    old value:  ''
    new value:  'foo'
main::(watchpoint-test.pl:3):            $onto{'PROBLEM'}{'mappings'} = "foo" if $c == 99;
  DB<2> p $c
100
  DB<3> c
Watchpoint 0:   $onto{'PROBLEM'}{'mappings'} changed:
    old value:  'foo'
    new value:  ''
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> q