If you want to change the picture, then do

$button->configure(-image => $photo_object);
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where $photo_object is constructed with $mw->Photo or $mw->Pixmap. And do not forget to delete unused photo objects, otherwise you'll got a memory leak. Another alternative is to change the filename in the photo object:

#!/usr/bin/perl
use Tk;
$mw = tkinit;
$p = $mw->Photo(-file => Tk->findINC("Xcamel.gif"));
$b = $mw->Button(-image => $p,
         -command => sub {
             $p->configure(-file => Tk->findINC("openfolder.xpm"));
         })->pack;
MainLoop;
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