if (@ARGV && $ARGV[0] ne "-u") {
  print "\nInvalid option.\n";
  die "usage: lamencrypt.pl [option] (-u decrypts)\n";
}

print "\nEnter string to translate:     ";
chomp(my $x = <STDIN>);
my @output = foo($x);

  sub foo {
  my $x = shift;
  if (@ARGV && @ARGV[0] =~ /-u/) {
    vec($x, $_ - 1, 8) -= $_ for 1..length($x);
    }
    else {
    vec($x, $_ - 1, 8) += $_ for 1..length($x);
    }
  return $x;
  }

print "\nLamely encrypted:              ";
print "@output\n";
[download]

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

Can anyone tell me why that didn't work?

Because "EXPR for EXPR" is not an EXPR, but a STATEMENT. And question-colon selects EXPRs, not STATEMENTs.

That's also why "EXPR for EXPR for EXPR" doesn't work.

-- Randal L. Schwartz, Perl hacker

You could do:

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

Also, as long as you're doing this for education, take a look at Getopt::Std, and that ARGV logic can be simplified further...