You will have to fetch the resource via HTTP through a proxy script and then serve the resource under your website content via HTTPS. There are examples in (for example) LWP::UserAgent::Paranoid and LWPx::ParanoidAgent.

Note that your script needs to be very paranoid to prevent Javascript data from being served in an image context etc. - I think the popular approach nowadays is to serve such content from a separate domain or subdomain and have the forum page supply the appropriate CORS headers.