With regular boolean tests, you can use DeMorgan's identities and other manipulations to reverse the test. In Perl, you have if/unless pairs to reverse the sense of the test.

We have || returning the actual value of the first true thing, so we can easily apply a default if something is not specified, and // improves upon that.

But I want the opposite: If some value is undefined, I want to stop there and return undef from the function. Only if it's defined do I want to keep going and return the result of subsequent processing, such as another function call.

my $x= ...;
return undef unless defined $x;
return morefoo($x);

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I can't help but find that annoying and ugly, as there should be a sleek way of writing it that's as nice as return $this // $that; given that so many such succinct things are indeed available. So what is a good idiom? I could certainly write the above as one statement but I still mention $x twice.

In reply to What's the opposite of // (err) operator? by John M. Dlugosz

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