mitmproxy-bin (free and open source interactive HTTPS proxy)

mitmproxy-bin is your swiss-army knife for debugging, testing, privacy
measurements, and penetration testing. It can be used to intercept,
inspect, modify and replay web traffic such as HTTP/1, HTTP/2,
WebSockets, or any other SSL/TLS-protected protocols. You can prettify
and decode a variety of message types ranging from HTML to Protobuf,
intercept specific messages on-the-fly, modify them before they reach
their destination, and replay them to a client or server later on.

This is just a repack of the prebuilt binaries from the mitmproxy
site, with some documentation added. Since upstream only releases
x86_64 binaries, this will *not* work on 32-bit x86 (or ARM, etc)
Slackware... although you can build an x86_64 package on a 32-bit host
and install it on a 64-bit one, if you like.