I came across this thread whilst looking to do something more complex. Essentially the most general regex case where the replacement might have $1 etc in it. Thought I'd post my solution, along with the comments I put in my code (note that the e(vals) aren't a concern for my usage, but others might want to guard against injection):

# We want to replace the $nth match of $regex in $value
# with $replacement

# OK, so doing the nth regex replacement in a string just
# isn't going to be that readable, but at least it's
# concise. I'll try to explain:
# As we go through the regex matches we want to count
# down from $nth to zero being when we want to replace.
# $& is just what we matched, so we'll write that when
# we're not replacing ie when $nth!=0.
# qq{} is just quotation that saves us escaping ' or " in
# the quotation string.
# So here we're creating a string where $replacement is
# parsed into quotations eg --$nth ? $& : qq{new text $1}

$rep_eval = "--\$nth ? \$& : qq{$replacement}";

# Then here we have our usual regex search and replace
# which we perform g(lobally) and e(val) twice.
# 1st e(val) parses $rep_eval, and the 2nd evaluates the
# "<if> ? <true> : <false>" expression for each match.

$value    =~ s/$regex/$rep_eval/gee;
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