open (IMG , '>out.png') or die $!;
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If you were trying

open (IMG , '>$$.png') or die $!;
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You would have to use double quotes so $$ is interpolated. You didn't show the example of you using the pid for the filename so I could off on your problem.

As for creating a unique filename, I prefer the quick and dirty way

my $filename = $$ . time() . '.png';
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