It works great, but do you really need the sub prototype? Removing the prototype works just as well.

#sub sobelEdgeDetection($);

sub sobelEdgeDetection { }
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Also, I don't know the exact differences, but the IM charcoal method produces similar output, but quicker. :-)

#!/usr/bin/perl
use warnings;
use strict;
use Image::Magick;

my $file = shift or die "need a file\n";;

my $p = new Image::Magick;
$p->Read($file);

$p->Charcoal('0x1');

$p->Write($0.'.jpg');

exit;
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