use Benchmark;

$string = 'aa ab ac ad ae af ' x 10000 . ' ff';
@string = split /\s/, $string;

$regex = <<'CODE';
&regex;
sub regex {
    do{return 0 unless /^a[a-z]$/} for @string;
    return 1;
}
CODE

$hash = <<'CODE';
$hash{$_}++ for aa..az;
&hash;
sub hash {
    do{return 0 unless defined $hash{$_}} for @string;
    return 1;
}
CODE

timethese ( 100, { 'regex' => $regex, 'hash' => $hash } );

__END__

Benchmark: timing 100 iterations of hash, regex...
      hash: 21 wallclock secs (20.37 usr +  0.00 sys = 20.37 CPU) @  4.91/s (n=100)
     regex: 45 wallclock secs (45.10 usr +  0.00 sys = 45.10 CPU) @  2.22/s (n=100)