my %button;

for my $label (qw/BUTTON1 BUTTON2 BUTTON3/){
   $button{$label} = $mainwindow->Button(-text => $label)->pack();
}
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Please make sure to Dominus' archived newsposts, part one, two and three on Why it's stupid to 'use a variable as a variable name'. Also, I suggest you make use strict; use warnings; a habit.

Makeshifts last the longest.