At that very moment when being defined, or later, when called? Have you tried an anonymous sub? 
-command => sub { $number=$number==0||$equals!=0?$input:$number.$input
+ },

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Update: See the docs @ Tk::Button and follow the "Perl/Tk callback" link.

In reply to Re: Making a button execute a command: Tk by eff_i_g
in thread Making a button execute a command: Tk by Andrew_Levenson

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