Check out "Tk::bind". It'll go something like this:

$mw->bind( 'Tk::Entry', '<Enter>', \&send_to_shell );
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(or it can be called as a method of the specific Entry widget where you want this to happen). I borrowed that line from a Tk "cryptogram" script that I posted here several years ago (it still works): Tk cryptogram (no, not cryptography)