Actually this is not a Perl question, but still I'll offer my 2 cents.
In order to achieve what you want, you'll have to play with javascript and capture key down or key press events. Get the code for the key that was pressed and execute the action for the second button if the key is Enter.

In reply to Re: Changing the default button in a CGI perl script by olus
in thread Changing the default button in a CGI perl script by Anonymous Monk

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