#!/usr/bin/perl

use strict;
use warnings;

my $access_log = $ARGV[0] || '/var/log/apache2/access.log';
my %user_agents = ();

if (open FILE,'<',"$access_log") {
    while (<FILE>) {
        my $line = $_;
        if (my ($ua) = ($line =~ /\"([^"]+)\"$/)) {
            $user_agents{$ua}++;
        }
    }
    close FILE;
} else {
    print STDERR "could not open \'$access_log\': $!\n";
    exit;
}

my @sorted = sort { $user_agents{$b} <=> $user_agents{$a} } keys %user_agents;

foreach my $user_agent (@sorted) {
    print "$user_agent: $user_agents{$user_agent}\n";
}