while (/(pattern)/g) {
  my $startpos = pos -= length $1;
  substr($_, $startpos, length $1, do_something($1));
  pos = $startpos;
}

##</code><code>##

$_ = "aaa aaa aaa aaa";
while (/(aa)/g) {
  pos $_ -= length $1;
  substr($_, pos, length $1, "ba");
}
print;