Here’s how to determine what modules have been installed after the original Perl installation, hence showing those modules not part of the core installation. Type this from a command line:

perldoc perllocal

Often, I need to compare two variables but I don’t know if they are numbers or strings. I need to know the type so I can pick the proper comparison operators, i.e., == or eq, > or gt.
So, here’s how to do it:

~$x ne ~"$x" ? 'numeric' : 'string'

~ is the bitwise negation operator (see perlop). It does not negate integers and strings in the same way.

Examples:

$ perl -e 'print ~1, "\n"'
4294967294
$ perl -e 'print ~"1", "\n"'
Œ

(character which ASCII code is 255 - ord(’1′))