Your questions makes enough sense.

To answer question one, consult your web server documentation for "Server-Side Includes". If you're using Apache, look here.

To run another perl script (or another program) from within a perl script, capturing the output, read perlipc.

To use a subroutine that exists in another script, you're going to have to read up on how to construct Perl modules (pacakges), or how the require statement works. It's not trivial to reach into another script and invoke another subroutine, but that's a part of the path to Perl knowledge yet ahead of you. Why spoil the fun here?