The documentation of keys says: "In scalar context, returns the number of keys or indices." Based on this I would say that keys has a shortcut to return the number of keys w/o building an array first. A little experiment seems to confirm this: 

use strict;
use warnings;
use Time::HiRes qw/time/;

my %hash;
my $n = 1000000;
@hash{1..$n} = 1 x $n;

my $s = time();
my @k = keys %hash;
print scalar @k, ": ";
print time()-$s, "\n";

$s = time();
print scalar keys %hash, ": ";
print time()-$s, "\n";

[download]

The second version is vastly faster.

In reply to Re: performance of counting keys in a big hash by hdb
in thread performance of counting keys in a big hash by xufengnju

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