use strict;
use WWW::Mechanize;
use YAML qw(DumpFile LoadFile freeze thaw);

# init a mech instance
my $mech = WWW::Mechanize->new(cookie_jar => {ignore_discard => 0}); # save even browser-lifetime cookies

# on first instance...
if(! -f 'stored_mech.dat') {
    # get, freeze, save cookie jar, and store
    $mech->get('http://localhost/mech-form.php');
    $mech->cookie_jar->save('cookies.dat');
    DumpFile('stored_mech.dat', [freeze($mech)]);
    exit;
}
# on subsequent instance...
else {
    # retrieve, thaw
    my $stored = LoadFile('stored_mech.dat');
    $mech = thaw($stored->[0]);
    # re-get page and reload cookies
    $mech->get('http://localhost/mech-form.php');
    $mech->cookie_jar->clear();
    $mech->cookie_jar->load('cookies.dat');
    # submit form with (presumably) original session
    $mech->submit_form(form_number=>1);
}