It has two distinct meanings:

• behind-the-scenes: doing $x++ when $x is undef returns 0, not undef, in a magical sort of way (the ++ causes it to return 0)
• highly advanced technology^*: $! is both a number and a string. printf "%d: %s", $! = 10, $!;

(* any sufficiently advanced technology is indistinguishable from magic)

japhy -- Perl and Regex Hacker