A nitpick: context refers to the calling environment and how any return values will be assigned. If those results are not utilized to populate a single variable (scalar context) or a list of variables (list context) but are merely tossed away, then the context is void (cf. wantarray).

In this case, the substitution does occur in void context, however that's not why $_ is assumed to be the target; rather it's because the subroutine doesn't explicitly tell s/// what to operate on and thus perl fills that value in for you.