Two things:

1) that while loop should read:

   while (<FS>) { $OP .= $_ }
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Note the all-important ".=" instead of "=" (probably just a mistype on your part. :)

2) your opens should always check for error messages, and you should get faster results slurping the whole file in as one string, like so:

    sub readfile {
        my $OP;
        open FS, $_[0] or die "Can't open $_[0]: $!";
        {
            local $/ = undef;
            $OP = <FS>;
        }
        close FS or die "Can't close $_[0]: $!";
        return $OP;
    }
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