You can set a watch-expression (an expression which triggers a halt if its value changes) with the "w" command, as in the following example:

DB<1> w $count
DB<<3>> L
Watch-expressions:
$count

Now that the watch-expression has been set, you can see how it works by writing some code to alter the value of the $count variable. The debugger will display a message every time the variable's value changes.

DB<2> for ($count=0; $count<=10; $count++) { print "Hello!"; };
Watchpoint 0: $count changed:
old value: undef
new value: '0'
DB<<3>> s
main::((eval 13)[/usr/lib/perl5/5.8.0/perl5db.pl:17]:2):
2: for ($count=0; $count<=10; $count++) { print "Hello!"; };;
DB<<3>>
Watchpoint 0: $count changed:
old value: '0'
new value: '1'
main::((eval 13)[/usr/lib/perl5/5.8.0/perl5db.pl:17]:2):
2: for ($count=0; $count<=10; $count++) { print "Hello!"; };;
DB<<3>> Hello!
main::((eval 13)[/usr/lib/perl5/5.8.0/perl5db.pl:17]:2):
2: for ($count=0; $count<=10; $count++) { print "Hello!"; };;
DB<<3>> s
Watchpoint 0: $count changed:
old value: '1'
new value: '2'
for ($count=0; $count<=10; $count++) { print "Hello!"; };

Watch-expressions can be deleted with the "W" command; either specify the expression

DB<<3>> W $count

or delete all currently-set expressions with the * wildcard.

DB<<4>> W *
Deleting all watch expressions ...

Note that adding watch-expressions can result in a performance penalty, so you should try and restrict yourself to not more than three or four at any given time.