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

my $mech = WWW::Mechanize->new( autocheck => 1 );
$mech->get( 'http://www.perlmonks.org/' );

my $response = $mech->response();
for my $key ( $response->header_field_names() ) {
    print $key, " : ", $response->header( $key ), "\n";
}
# Or if you know the header key in advance
my $header = $mech->response()->header( 'key' );