This thread has turned into the "Perl Quiz of the Week" #1! ;-)

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')
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should return

    "fish, dogs, and carrots"
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dominus' post-mordem analysis of this problem can be found here. He offers several solutions and some discussion of why (and why not) to use them.