jmo@foo:~> ls /some/small/dir|wc -l
322
jmo@foo:~> ls /some/large/dir|wc -l
12337
jmo@foo:~> perl ls.pl 
Benchmark: timing 5000 iterations of allready opendir on small dir, ls on small dir, opening dir on small dir...
allready opendir on small dir:  3 wallclock secs ( 2.83 usr +  0.50 sys =  3.33 CPU) @ 1501.50/s (n=5000)
ls on small dir: 30 wallclock secs ( 0.60 usr  2.48 sys + 11.90 cusr 16.85 csys = 31.83 CPU) @ 1623.38/s (n=5000)
opening dir on small dir:  4 wallclock secs ( 3.31 usr +  0.61 sys =  3.92 CPU) @ 1275.51/s (n=5000)


Benchmark: timing 5000 iterations of allready opendir on large dir, ls on large dir, opening dir on large dir...
allready opendir on large dir: 102 wallclock secs (87.82 usr + 13.04 sys = 100.86 CPU) @ 49.57/s (n=5000)
ls on large dir: 57 wallclock secs ( 0.51 usr  1.99 sys + 27.62 cusr 27.09 csys = 57.21 CPU) @ 2000.00/s (n=5000)
opening dir on large dir: 101 wallclock secs (88.11 usr + 12.37 sys = 100.48 CPU) @ 49.76/s (n=5000)

##</code><code>##

use Benchmark;
use strict;
my $dir = '/some/small/dir';
my $file = 'somefile*';
opendir (F, $dir);

timethese(5000, {'ls on small dir' => \&foo, 'allready opendir on small dir' => \&bar, 'opening dir on small dir' => \&baz});

$dir = '/some/large/dir';
$file = 'somefile*';
print "\n\n\n";
closedir F;
opendir (F, $dir);
timethese(5000, {'ls on large dir' => \&foo, 'allready opendir on large dir' => \&bar, 'opening dir on large dir' => \&baz});
closedir F;

sub foo
{
    my $a = `ls $dir/$file &>/dev/null`;
}

sub bar
{
    my (@files) = (grep (/$file/, readdir(F)));
    rewinddir (F);
}


sub baz
{
    opendir (DIR, $dir);
    my (@files) = (grep (/$file/, readdir(DIR)));
    closedir F;

}