in reply to
Re^4: Can't locate WWW/Mechanize/Firefox.pm in @INC
in thread Can't locate WWW/Mechanize/Firefox.pm in @INC
path with no arguments from your command prompt will show the existing path.
you can add to it, change it, etc. Your "2." seems to say that the Oracle @inc may be higher (earlier) in your path than the 5.16 path... or that 5.16 doesn't have a proper path to your modules (which should be in a series of directories subordinate to the dir named Perl.
As to an additional instance of Perl, you've answered that with perl -v which says immediately that 5.16 was executing. So, you've already checked that.
But we still don't have quite enough info to give you an Rx for a cure.