I've also been using the debugger quite often recently because of inherited code. The ones I use most:

1. break at a line number: b 234
2. break at a function : b foo
3. simply change the value of a variable, e.g, the script is processing all files a directory, so it got a list at the beginning, I only want to look at a particular file, I can simply break after gettting the list, then change the list to contain that particular file.
4. dump a data structure: use Data::Dumper; p Dumper($foo)