Update: sorry, I missed jsprat's previous answer, so this post is a poor dupe :-)

this may not be properly what you're looking for, but Dominus's first Quiz of the Week was very similar.

the goal was to:

Write a subroutine, 'subst', which gets a string argument, $s. It should search $s and replace any occurrences of "$1" with the current value of $1, any occurrences of "$2" with the current value of $2, and so on.

For example, if $1, $2, and $3 happen to be "dogs", "fish" and "carrots", then

        subst('$2, $1 and $3')

should return

        "fish, dogs, and carrots"

you can find the original question along with MJD's solution and discussion here.

hope this helps (or entertains at least :-).

oh, by the way: Dominus++ for the marvelous QOTW!

cheers,
Aldo

King of Laziness, Wizard of Impatience, Lord of Hubris