If you start wish (a tk shell) with the following command, you can see the KeySym name that Tk receives. In this case the keypad return key has the keysym 'KP_Enter' while the normal return key has the keysym 'Return'

wish
% bind . <KeyPress> {puts %K}
KP_Enter
Return
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This can be used as in the question "10.5. How do I bind keyboard keys?" of the perl/tk FAQ http://www.faqs.org/faqs/perl-faq/ptk-faq/part2/

PS: This information and more can be found on the web by searching for 'tk keyboard mapping' and 'perl/tk key bindings'.