82 chars (updated). Returns 1 for valid, undef otherwise. Your solution seems to be out by one for input that has an odd number of digits.

sub luhn {
    $r=$t=0;
    split//,pop;
    $r.=$_[$_]*(1+($_+$#_)%2)for+0..@_;
    $r=~s/(.)/$t+=$1/eg;
    !chop$t
}
[download]

Update: Here is another non-reverse variation at 72 chars. Still a mile behind chipmunk however.

sub luhn2 {
    $_=$i=0;
    split//,pop;
    $_.=(1+$i++%2)*pop while@_;
    s/(.)/+$1/g;
    $_=eval;
    !chop
}
[download]

--
John.