I don't know what you will be using it for, but if you just want to run a couple commands, then a sub-shell may work for you; just exit from the subshell, when you're done...

# I use bash, but you can put csh here, if you want
# I added "--norc" so that PS1 is not overridden
$ perl -e '$ENV{SQUIRREL}="brown";$ENV{PS1}="subshell> ";system("bash"
+, "--norc")'
subshell> echo $SQUIRREL
brown
subshell> exit
exit
$ echo $SQUIRREL

$
[download]

The problem here is that you may forget that you're in a sub-shell and spawn several subshells. One way to address that is to add an environment variable to your script, say, for example, $ENV{MYSCRIPTISRUNNING}="YES", then you could simply exit if this env variable exists, preventing multiple subshells. This is not foolproof, but it's a start.

N.B. This creates a sub-shell, and the env changes will be lost once you exit all the way out back to your original shell.

HTH.