\z, as proposed by haukex might be the most appropriate solution: it only matches at the absolute end of the string, so both greedy and non greedy variants do what you want.

$p = substr($s, @-[1], (@+[1] - @-[1])); is a strange way to write $p = $1; you know you can access captures with $X with X the number of the of the capture right?

Also, rather than trying to make your output more explicit with enclosing [ and string length, I strongly advise you to use Data::Dump (or the Core Data::Dumper, but you have to set $Data::Dumper::useqq = 1; for the more explicit version) for debugging (it will take care or invisible, or look-alike characters for you, and works well with refs and structures)

use Data::Dump 'pp';
$s = "abc\n";
if ($s =~ /(ab.*?)\z/s) {
    pp($1);
}
if ($s =~ /(ab.*)\z/s) {
    pp($1);
}

__END__
"abc\n"
"abc\n"
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