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!";
}
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prints nothing.