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if ($s =~ /^a(?!.*ab).*b$/) {

use strict;
use warnings;
...
   ? qq{$_: matched\n}
   : qq{$_: unmatched\n}
   for @strings;

a000000000b: matched
a0a0000b00b: matched
a0000ab000b: unmatched
a00000000ab: matched