Not sure if this is exactly what you want, but you can set an action to happen before each debugger prompt with the "<" command.

So, supposing you wanted to watch $foo, you could say
< print "foo is $foo\n";

If you want to set a multiple-line command to run, put backslashes at the end of the lines to continue to another line.

Update: ferrency's method works much better :) Anybody know why 'W' doesn't seem to be in Perl in a Nutshell? Wish I had known about that earlier....