I like the idea a lot. Performance, although not to be neglected when thinking about how a feature will affect the average case, is not so much motivation to me as the fact that this provides a clean way to do it Once And Only Once. Right now, there is no way to achieve that for exists tests. For multilevel lookups you can roll up the lookup for most of the path which gets close, but no cigar: 
exists $_->{$client} and return $_->{$client}
    for $export_def->{'..client_default_version'};

[download]
But there's just no way around the duplicate mention of $client. And just as I was typing this code, I forgot the sigil in the second lookup and only by chance caught it a few seconds before submitting, which demonstrates why Once And Only Once is a good idea. Had I made this mistake in actual code, it would have broken silently and possibly left me to scratch my head for a long time.

Makeshifts last the longest.

In reply to Re: optimization - exists should return a reference by Aristotle
in thread optimization - exists should return a reference by John M. Dlugosz

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