"The source-code to any installed package can also be found in the library directories of your computer. The PERL5LIB environment-variable (or its equivalent, found in some control-panel, in Windows), will tell you where."

No, that's wrong!

@INC contains the directories. These directories include whatever's in $PERL5LIB if it's been set or in $PERLLIB if that's been set.

This is documented in perlrun: ENVIRONMENT:

PERL5LIB

A list of directories in which to look for Perl library files before looking in the standard library and the current directory.
...
If PERL5LIB is not defined, PERLLIB is used. ...

PERLLIB

A list of directories in which to look for Perl library files before looking in the standard library and the current directory. If PERL5LIB is defined, PERLLIB is not used. ...

You can confirm this with perl -V (look under %ENV: and @INC: at the end of the output).

Please check your information before posting.