The desired behavior can be forced by using //iaa

use v5.18.0;
use warnings;

my $pattern = "(?<!ss)abc";
my $regex = qr/$pattern/iaa;
say 'ok' if 'ssqabc' =~ $regex;
say 'ok' if 'ssabc'  !~ $regex;
say 'ok' if 'ssßabc' =~ $regex;
[download]

From perlre:

To forbid ASCII/non-ASCII matches (like "k" with "\N{KELVIN SIGN}"), specify the "a" twice, for example "/aai" or "/aia". (The first occurrence of "a" restricts the "\d", etc., and the second occurrence adds the "/i" restrictions.) But, note that code points outside the ASCII range will use Unicode rules for "/i" matching, so the modifier doesn't really restrict things to just ASCII; it just forbids the intermixing of ASCII and non-ASCII.