Thanks to ikegami (!), aquarium & dasgar for politely pointing me in the right direction. Thank you ww (assuming, that is, it is not some automatically generated reply which is triggered by the pattern /how should i/ig ) for warning me from the undesirable consequences my message may inflict (the worst of all is the re-reading the "how to pose a question" tutorial).

though not a one liner the following is simple to understand and does the trick

$_='nor neither nor'; 
my $cnt = 0;
while ((/(.*?)nor/ig) ) { # =================== coarse match
    printf ("\n");
    printf (STDOUT qq/\$\`:%s\n/,$`);
    printf (STDOUT qq/\$\&:%s\n/,$&);
    printf (STDOUT qq/\$\':%s\n/,$');
    if ($1 !~ /\bneither\b/i){ # =========== finer match
        printf ("match no. %d: ",$cnt++);
        printf ("matched ok\n");
    } 
    else {
        printf ("match no. %d: ",$cnt++);
        printf ("no match\n");
    };
}
[download]