What about something like:

#!/usr/bin/perl

use strict;
use warnings;
use Time::HiRes qw(gettimeofday tv_interval);

my $testdir = $ARGV[0];
my @filelist;

# Code example taken from Time::HiRes documentation
opendir(D, $testdir) or die $!;
    my $t0 = [gettimeofday];
    @filelist = readdir(D) or die $!;
    my $t1 = [gettimeofday];
closedir D;

my $t0_t1 = tv_interval $t0, $t1;

print q{Scan time for directory }, 
  $testdir, 
  q{ : },
  $t0_t1,
  qq{ seconds\n}; 
print q{Total entries in directory },
  $testdir,
  q{ : },
  scalar @filelist,
  qq{\n};
[download]

Example output:

$ perl readdir-test.pl .
Scan time for directory . : 3.5e-05 seconds
Total entries in directory . : 4
$ perl readdir-test.pl test/
Scan time for directory test/ : 0.113722 seconds
Total entries in directory test/ : 68115
$ perl readdir-test.pl test2/
Scan time for directory test2/ : 1.475231 seconds
Total entries in directory test2/ : 412483
[download]

HTH.

Updated: 14-Aug-2007
Added example for directory test/ containing 412483 entries.