The following may help:

#!/usr/bin/perl
use warnings;
use strict;
use WWW::Mechanize;

my $url = 'http://perlmonks.org/?';
my $mech = WWW::Mechanize->new();
my $response = $mech->head($url);
my $modDate = $response->header('last-modified');

if (defined $modDate) {
    print "Modification date: $modDate\n";
} else {
    print "Last modification date not available\n";
}
[download]

Prints:

Modification date: Wed, 25 May 2016 07:45:13 GMT
[download]