Scripts are loaded and unloaded with /perl, /python, /ruby and /lua commands (type /help in WeeChat for help about commands).
Examples:
Load a Perl script: /perl load /tmp/test.pl
List all loaded Perl scripts: /perl
Load a Python script: /python load /tmp/test.py
List all loaded Python scripts: /python
Load a Ruby script: /ruby load /tmp/test.rb
List all loaded Ruby scripts: /ruby
Load a Lua script: /lua load /tmp/test.lua
List all loaded Lua scripts: /lua