$ perl -E '
> say qq{$_ - @{ [ m{a(?!.*b)} ? q{Match} : q{No match} ] }} for (
>    q{Have you any fish},
>    q{What a nice broom},
>    q{Fishing for trout},
>    );'
Have you any fish - Match
What a nice broom - No match
Fishing for trout - No match
$