Perl 'Expert' 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")

should return

        "fish, dogs, and carrots"

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