batkins,
Thinking out loud here...

If the options were not going to be parsed then no. The _ usually preceeds a variable or a sub in an OO module to indicate it is private and not meant for public consumption. A dash is usually used as a command line argument.

So, if the module were going to be passing your arguments to something like Getopt::Long for instance, it would make perfect sense. I haven't played with Perl/Tk yet, but that would be my guess.

Cheers - L~R