It can be I guess, but it will work just as well as \1.
Outside of the s/// operator I must use $1 but while I am inside it I can use $1 or \1, \2 or $2...
Update: Even though it works just as well, as per the perlre: "this is a dirty habit to get into" (as it was grandfathered in for sed fans). Sometimes a new trick is not so new nor wise to use without restraint (in this case probably never lest I be working with sed).