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if (@ARGV && $ARGV[0] ne "-u") {
  print "\nInvalid option.\n";
...

print "\nLamely encrypted:              ";
print "@output\n";

@ARGV && @ARGV[0] =~ /-u/ ? vec($x, $_ -1, 8) -= $_ for 1..length($x) 
+: vec($x, $_ -1, 8) += $_ for 1..length($x);