Try Prima, it supports non-rectangular windows - on Win32 natively, on X11 if you've shape extension compiled in the X server:

use Prima qw(Application);

my $icon = Prima::Image-> create(
   width  => 300,
   height => 300,
);
$icon-> begin_paint;
$icon-> color( cl::Black);
$icon-> bar( 0, 0, 300, 300);
$icon-> color( cl::White);
$icon-> fillpoly([ 
  5, 5, 
  250, 5,
  5, 200,
  100, 100,
  5,5
]);
$icon-> end_paint;

Prima::Widget-> create(
   shape => $icon,
   size  => [ 300, 300],   # fixed size
   onMouseDown => sub { $::application-> close }, # click to exit
);

run Prima;
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