I can't get an response via LWP::UserAgent on my requests.

What does that mean? I'm under the impression that request returns a response for both successes and failures.

And how did you ascertain this conclusion?

I thought that might help if there is a scoping error

You have tons of those. use strict; will reveal them.

My basic single threaded script works fine

A quick look at LWP's and HTML-Parser's documentation shows no indication that they are thread-safe.

LWP appears to be completely written in Perl, so it's *probably* safe.

HTML-Parser, on the other hand, is partially written in C. It's much more likely for a C module to not be thread-safe (since variables are shared by all threads). What happens if you remove all references to HTML::LinkExtor (temporarily)?