Highly ASCII-specific, and inefficient, but perhaps mildly interesting. Appears to work, though no absolute guarantees are made of course. Using tr/// is kind of a wart, though, hmm.

sub contains_alpha
{
    use bytes;
    my ($string) = @_;
    my $len = length($string .= "");
    my @b = map{$string & (chr(1<<$_) x $len)} (0..7);
    tr/\x01-\xff/\x01/ for @b;
    my $result =
      (~$b[7]) & ($b[6]) &
      (($b[4] & ~($b[3] & (($b[1] & $b[0]) | $b[2])))
       | ((~$b[4]) & ($b[3] | $b[2] | $b[1] | $b[0])))
      ;
    $result =~ tr/\x00//d;
    length($result);
}
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