From the sounds of it, you can't actually pass an arrayref :(

Sure you can. Here's a simple example:

#!/usr/bin/perl
use strict;
use warnings;
use LWP;

my $url = 'http://localhost/echo.cgi';
my $ua = LWP::UserAgent->new;
my $res = $ua->post ($url, { foo => ['bar', 'baz'] });
print $res->as_string;
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Throw that at your handler and it will tell you that param "foo" has two values which are "bar" and "baz". If it doesn't, that's a bug in your handler and you'll need to head on over to PHPMonks to sort that out.