I suspect that your problem is in the indexing - you've used rand(20) and rand(5) although there are 19 consonants and 6 vowels. Here's my version of your code:

print "gimme a seed: ";
my $s = <STDIN>;
my @chars = qw(b c d f g h j k l m n p r s t v w x z);
my @vowels = qw(a e i o u y);
my $length = 4; #Should be (length of password)/2

srand ($s ^ time);

print $chars[int(rand(@chars))], $vowels[int(rand(@vowels))] while($le
+ngth--);
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update: Thanks to chipmunk and Aighearach change int(rand(length(@vowels))) to <int(rand(@vowels)) - As Aighearach says, "Yay! for context!" :) hth, bent