To answer your second question – Perl source-code packages specified by "use" will be picked up while the program is being compiled and will then remain as part of the total compiled program (which is what the interpreter actually runs). Dynamic loading of Perl source-code packages thereafter, "on the fly," is possible but is much more rarely used. The XS-loader that you make reference to is a mechanism for loading of dynamic binary modules (".so" and ".dll") from the host operating system ... used where the Perl module is all-or-part a "wrapper" for the functions of that library or where a binary implementation is used for speed. It is not used for Perl source-code modules.