very nice. i played around with the same script for a while before discovering your module, but your version is much better. you can get a small speed increase (~20% or so) by generating an array mapping the transformation instead of computing it every time through the loop. before your while loop, add:

my @ymap = map { ($_-42+256)%256 } (0..255);
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then replace:

179:        push @uline, ($bytes[$i]-42+256)%256;
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with:

push @uline, $ymap[$bytes[$i]];
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