How can all these different scripting languages work smoothly with ASP?
The answer is Microsoft's ActiveX Scripting standard. ActiveX Scripting is one of Microsoft's many ActiveX technologies.
ActiveX Scripting says simply this: if you want to develop a scripting language, make sure it supports the standard, and Microsoft will guarantee that it works with Internet Explorer and ASP (among other things).
Microsoft didn't decide to include Perl as a scripting language, someone else used ActiveX Scripting to add Perl capability.