That's a very good question, actually.

Here's one straightforward way to do it, assuming you have a package called my_functions, and you want to call my_functions::subbie_1:

$ent->bind('<Return>'=> sub { my_functions::subbie_1() });
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What the above code does is to create an anonymous (ie. unnamed) subroutine, which is a pointer to a subroutine that gets invoked whenever the Return key is struck.

You can also call subbie_1 with arguments if need be, of course.