$-[0] is the offset of the start of the last successful match. $-[n] is the offset of the start of the substring matched by n-th subpattern, or undef if the subpattern did not match.

edit

It's hard to tell which result you expect, but the way pos is changed after replacing a zero length assertion is tricky. It's meant to avoid endless loops.

use v5.12;
use warnings;
use Data::Dump 'dd';


$_ = 'aa';
s/(?=(a))(?{say "\$1='$1' \@-=(@-) pos=",pos})/b/g;
dd [$1];
dd \@-;
dd $_;

__END__
$1='a' @-=(0 0) pos=0
$1='a' @-=(0 0) pos=0
$1='a' @-=(1 1) pos=1
$1='a' @-=(1 1) pos=1
["a"]
[1]
"baba"
[download]

update

this may shed some light

use v5.12;
use warnings;
use Data::Dump 'dd';


$_ = 'ab';
s/(?=([ab]))(?{say "\$1='$1' \@-=(@-) pos=",pos})/X/g;
dd [$1];
dd \@-;
dd $_;
__END__
$1='a' @-=(0 0) pos=0
$1='a' @-=(0 0) pos=0
$1='b' @-=(1 1) pos=1
$1='b' @-=(1 1) pos=1
["b"]
[1, 1]
"XaXb"
[download]