This got me a little farther, but now I need to call the subroutine when the button is clicked.

my $button1 = $mw->Button(-text => "18-Letters",
                   -command => sub {print OUTFILE "18L\n",\&onclick,}
                          ,)
                          ->pack(-side => "top");
sub onclick{
   $button1->configure(-background=>"yellow")
}
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