#!/usr/bin/perl
use Benchmark qw(:all);

%h_delete = %h_list = %h_undef = (0 .. 50000);

delete @h_delete{keys %h_delete};
%h_list = ();
undef %h_undef;

timethese 10000, {
    delete  => '1 for keys %h_delete',
    list    => '1 for keys %h_list',
    undef   => '1 for keys %h_undef',
    };

__END__

Benchmark: timing 10000 iterations of delete, list, undef...
    delete:  5 wallclock secs ( 2.43 usr +  0.00 sys =  2.43 CPU) @ 4115.23/s (n=10000)
      list:  5 wallclock secs ( 2.44 usr +  0.00 sys =  2.44 CPU) @ 4098.36/s (n=10000)
     undef:  0 wallclock secs ( 0.02 usr +  0.00 sys =  0.02 CPU) @ 500000.00/s (n=10000)
            (warning: too few iterations for a reliable count)