How about:

while (<>) { # the "x" operator does the right thing for negative valu
+es
  print $_.("X" x (130-length($_)));
}
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The only circumstance where Perl would fail on large files I can imagine is if the files are more than 2GB in size, and Perl wasn't compiled with large file support. But this loop should work even there (since it does not need to seek at all).