/(?(?{ whatever() })|(?!))/

Did I miss something?

Update: halley replied via /msg. His post was describing the same technique but gives more details for the benefit of on-lookers.

To clarify how I see this being used here's the code example from my patch to perlre:

@known_animal{ qw( cat dog fox horse rabbit rat ) } = (); 
@animals = /\b(\w++)(?{ exists $known_animal{$^N} }?)/g;
print "@animals\n";
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