Actually, there's no need to be careful or test for definedness.

List::Util::first() returns the result of the code block, in this case the boolean return of the exists function. The values associated with the keys never come into it.

use List::Util qw[ first ];;
my $a = { a=>0, b=>0, c=>0 }; 
my $b = { c=>0, d=>0, e=>0 };

if( first{ exists $b->{ $_ } } keys %$a ) { 
    print "Common keys!";
}
Common keys!
[download]

List::Util::first() returns the result of the code block, in this case the boolean return of the exists function.

i think it returns the key it's iterating over, just like grep works. from the docs: "first returns the first element where the result from BLOCK is a true value."

use List::Util qw[ first ];
my $a = { 0 => 0 }; 
my $b = { 0 => 0 };

if( first{ exists $b->{ $_ } } keys %$a ) { 
    print "Common keys!";
}
[download]

prints nothing.

Of course it does. Values of keys vs. values associated with keys. Sometimes I can be semantically blind. (Ie. plain wrong!)

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