This is probably a little simpler and a lot safer:

$had_bad_characters = $user =~ s/\W//g;

# Safer still (since what's defined as a 'word character' could change
+ based on locale/Unicode (?))
$user =~ s/[^a-zA-Z_-]//g;   # Explicitely define what we want to ACCE
+PT as valid
[download]

Generally the secure approach involves defining what is acceptable and disallowing everything else, not trying to filter out what we know/anticipate to be bad, because stuff frequently slips through.