Inline debugging is really, really helpful in cases like this. You might also want to change the

print OUT $_;
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to

print ;
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to print your lines on the screen.

And for a sanity check, you might put something like:

print "\$in_file: $in_file \n";
print "\$out_file: $out_file \n";
print "\$pattern : $pattern\n";
print "\$replacement:  $replacement\n";
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before your open commands. Just to make sure you're opening the file(s) you think you're opening.