What problems do you have?   For me, it seems to work as specified:

Assuming the following program 

#!/usr/bin/perl
for my $c (1..1000) {
    $onto{'PROBLEM'}{'mappings'} = "foo" if $c == 99;
}

[download]

here's an example session: 

$ 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

[download]

Maybe you're using an old version of Perl - I dimly remember that commands were slightly different in 5.6.x (I think 'w' was 'W', but I could be wrong).

In reply to Re: Alas, watch-expression not obvious to this one by almut
in thread Alas, watch-expression not obvious to this one by throop

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.