/(?<!ss)/i

##</code><code>##

/(?<!ß|ss)/i

##</code><code>##

    ßtabc
   SSTABC
    STABC

##</code><code>##

/(?<!s)tart/i ;

/(?<![Ss])[Tt][Aa][Rr][Tt]/ ;

##</code><code>##

my $not_s= qr/(?<![Ss])/; # no explicit /i here, will never match inside ß
my $rest= qr/abc/i;
my $pattern= "$not_s$rest";

...