You could save-and-restore pos:

while (/(pattern)/g) {
  my $startpos = pos -= length $1;
  substr($_, $startpos, length $1, do_something($1));
  pos = $startpos;
}
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Basically open-coding the s///g operator with an extra last step to rescan what's already been seen.

-- Randal L. Schwartz, Perl hacker

---

update:

Whoa. Unnecessarily complicated there. This is simpler:

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

This prints "bba bba bba bba", because the "a" being replaced is allowed to match part of the next "aa" search, as expected.