https://perldoc.perl.org/perlvar#@-

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]

Cheers Rolf
(addicted to the 𐍀𐌴𐍂𐌻 Programming Language :)
Wikisyntax for the Monastery

In reply to Re: Global match and capture group in look-ahead == under-populated "@-"? (updated) by LanX
in thread Global match and capture group in look-ahead == under-populated "@-"? by Anonymous Monk

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