Not so much a trick in the debugger, but I like to use it as an "interactive Perl shell", using a command like perl -de 1. Of course, any expression other than -e 1 will work, I'm just used to that one.
For example, if you're wondering whether a particular regex will match, or whether your split will work as you intended, but you don't want to mess around with shell quotes, just:
$ perl -de 1
Loading DB routines from perl5db.pl version 1.25
Editor support available.
Enter h or `h h' for help, or `perldoc perldebug' for more help.
DB<1> $s = '12345'
DB<2> x split(/(?<=\d)/, $s)
DB<3> q # yup, it works
I believe I first read about this in Effective Perl Programming