You can apply the following to your perl installations:
I have two versions of tcltk installed on my system. Both binaries live in (in my case) /usr/bin/
wim@desktop1:~$ ls -l /usr/bin/tclsh*
lrwxrwxrwx 1 root root 23 2007-07-17 14:21 /usr/bin/tclsh -> /etc/alternatives/tclsh
-rwxr-xr-x 1 root root 3704 2008-02-25 09:20 /usr/bin/tclsh8.4
-rwxr-xr-x 1 root root 3704 2008-04-22 21:22 /usr/bin/tclsh8.5
The standard one is pointed to by tclsh which is linked to /etc/alternatives/tclsh which in turn (not shown) is linked to /usr/bin/tclsh8.4. If I want to use (from the command line) tclsh8.4, I can either type tclsh or tclsh8.4 . If I want to use tclsh8.5, I type tclsh8.5 . To change the default (8.4 to 8.5) one can simply change the symbolic link.
If I want to use it in a script, I'll have to change the shabang as mentioned by jamescondron.
if you want the default (whichever it is), /usr/bin/tclsh8.4
if you want tclsh8.4 specifically and /usr/bin/tclsh8.5
if you want tclsh8.5 specifically.