I'm not clear on which of several problems you're trying to solve, so I'll assume that you don't want to hard-code the path in scripts that you intend to distribute, where they might break if they're installed on a box where Perl lives somewhere else.

The common way I see people addressing this is by writing an install script that first finds (or guesses, or asks for) the path to Perl, and then fixes up the #! line on each executable script component.

A alternate trick from the path-less-traveled is to use   #!./perl -wT then make a symbolic link the proper perl executable. This doesn't work for CGIs on Win32, but then neither does -T in the #! line. Some will consider this approach less than wise.