PATH editing in a script

[Helmut Jarausch <jarausch@xxxxxxxxxxxxxxxxxxx>]

Hi,
does anybody know how to edit the PATH environment variable?
Something like 'vared' but in a script (like sed)

Problem:

Having two versions of (say) TeX I would like to replace the PATH to tex
in a user PATH environment string.

Like
s|/usr/local/lib/TeX|/usr/local/teTeX|

It's no problem to edit the PATH e.g. by Perl but these changes have to
persist whence the (Perl/Zsh) script finishes.

Thanks for any hints,
Helmut.