If what you mean by "decompilable" that folks can read your source code, then yes they can do that. With ActiveState's PerlApp, the source code is encrypted somewhat.

If you are building standalone executable files, then there is a big overhead to get Perl itself to begin with (there is a minimum size). The incremental growth rate depends upon how much source code that you include for Perl to compile and work with. That includes your code and modules that you "use".

This is not a "compile", it is more like packaging the Perl executable engine and the code that you need together as one unit so that it does not require Perl to already be resident. It is not platform independent like a Java jar file.