s/.../888${1}555-$2/ will do it for you. The braces around the variable name, specifically, are what you're looking for.
Jeff japhy Pinyan,
P.L., P.M., P.O.D, X.S.:
Perl,
regex,
and perlhacker How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart