You could also force stringification to get an even bigger speedup compared to calling pack on every hash access (if that's what you're doing), e.g. $x = "$x" or $_="$_" for @values;. Update: Added the pre-packed test with $p, which surprisingly is still not as fast as the stringified float $y. Update 2: By giving each test case its own hash instead of using one hash for all tests, now the test cases "y" and "p" perform roughly the same, as I would have originally expected (Update 3: However, across multiple runs of this benchmark, "y" still tends to outperform "p", often significantly). Update 4: If you set $x = 42.123456789, then "p" consistently outperforms "y", so I'd guess it has to do with the hash key length. (last update, I promise ;-) )

use Benchmark qw/ cmpthese /;
my $x = 42.0;
my $y = "$x";
my $p = pack 'F', $x;
my (%h1,%h2,%h3,%h4);
cmpthese( -1, {
    F => sub { $h1{ pack 'F', $x } = 1 },
    s => sub { $h2{ $x }           = 1 },
    y => sub { $h3{ $y }           = 1 },
    p => sub { $h4{ $p }           = 1 },
});

__END__

        Rate    s    F    p    y
s  4032984/s   -- -64% -84% -84%
F 11327209/s 181%   -- -54% -56%
p 24707825/s 513% 118%   --  -5%
y 25997751/s 545% 130%   5%   --
[download]