The Perl-interpreter is needed to run Perl-code.
Typically, the interpreter is written in C.
There are different versions of the interpreter, some for Linux, others for Windows.
As the Perl-script runs on top of the interpreter, it is platform-independent, but the interpreter itself isn't.

perl2exe, PAR and others extract the files from the interpreter which are needed to run the script and put it all together into some code-package.
That's the way, they make the program seemingly independent from the interpreter: They supply the interpreter-parts (taken from the installed version of the interpreter) together with the script.

They do not compile the Perl-script to C or something.
If you wonder, why they don't, take a look at this article:

http://www.perl.com/pub/2001/06/27/ctoperl.html

Cheers