use Benchmark qw(cmpthese countit);
use POSIX qw(strftime);
use constant TIME => 10000000;
use constant FORMAT => '%Y-%m-%d %H:%M';

my $trash;

sub plain_strftime {
    return strftime(FORMAT, localtime(shift));
}

my ($timecache, $strcache) = (0, &plain_strftime($timecache));

sub cache_strftime {
    my $t = shift;
    if ($t - $timecache > 60) {
	$timecache = $t;
	$strcache = &plain_strftime($timecache);
    }
    return $strcache;
}

cmpthese(TIME, {
    'plain' => sub { $trash = &plain_strftime(time); },
    'cache' => sub { $trash = &cache_strftime(time); },
});

__DATA__

laptop:~> monk.pl
Benchmark: timing 10000000 iterations of cache, plain...
     cache: 79 wallclock secs (67.66 usr +  3.69 sys = 71.35 CPU) @ 140154.17/s (n=10000000)
     plain: 282 wallclock secs (248.43 usr +  7.03 sys = 255.46 CPU) @ 39145.07/s (n=10000000)
          Rate plain cache
plain  39145/s    --  -72%
cache 140154/s  258%    --