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]