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C:\>perl -MDevel::Peek -e "$x = 0.5; qq/$x/; Dump $x"
SV = PVNV(0x38bb4) at 0xd217cc
...
  PV = 0xd16274 "0"\0
  CUR = 1
  LEN = 10

use Benchmark qw/ cmpthese /;
$x = 42.0;
...
    F => sub { $h{ pack 'F', $x } = 1 },
    s => sub { $h{ $x }           = 1 },
});

       Rate    s    F
s  344025/s   -- -86%
F 2490475/s 624%   --