#!/usr/bin/perl
use warnings;
use strict;

my @keys = 'a' .. 'z';

sub one_by_one {
    my %hash;
    undef $hash{$_} for @keys;
}

sub slice {
    my %hash;
    @hash{@keys} = ();
}

use Benchmark qw{ cmpthese };
cmpthese(-3, {
    one_by_one => \&one_by_one,
    slice      => \&slice,
});

__END__
               Rate one_by_one      slice
one_by_one 267946/s         --       -25%
slice      359541/s        34%         --