well I can't

    
use strict;
use warnings;



our $h={};

warn "init";

$h->{k1}{k2}{k3}=42;




warn "call sub";

change();


warn "set";



$h->{k1}{k2}{k3}=666;

warn "end";

sub change {
    eval '$h->{k1}{k2}{k3}=12';
}
[download]

Current directory is d:/tmp/pm/

Loading DB routines from perl5db.pl version 1.49_05
Editor support enabled.

Enter h or 'h h' for help, or 'perldoc perldebug' for more help.

  DB<3> L
  DB<3> w $h->{k1}{k2}{k3}
  DB<4> c
init at d:/tmp/pm/t_watch.pl line 8.
 at d:/tmp/pm/t_watch.pl line 8.
Watchpoint 0:    $h->{k1}{k2}{k3} changed:
    old value:    ''
    new value:    '42'
  DB<4> c
call sub at d:/tmp/pm/t_watch.pl line 15.
 at d:/tmp/pm/t_watch.pl line 15.
Watchpoint 0:    $h->{k1}{k2}{k3} changed:
    old value:    '42'
    new value:    '12'
  DB<4> c
set at d:/tmp/pm/t_watch.pl line 20.
 at d:/tmp/pm/t_watch.pl line 20.
Watchpoint 0:    $h->{k1}{k2}{k3} changed:
    old value:    '12'
    new value:    '666'
  DB<4> c
end at d:/tmp/pm/t_watch.pl line 26.
 at d:/tmp/pm/t_watch.pl line 26.
Watchpoint 0:    $h->{k1}{k2}{k3} changed:
    old value:    '666'
    new value:    ''
  DB<4>
[download]

UPDATE

expanded code