Using one hash reference rather than two in Perl

Question

Is there some way in Perl that I can mention k just once on the second line:

my %k = (a=>1, b=>2, c=>undef);
say for grep{!$k{$_}} keys %k;

Answer 1

Make use of mapp and grepp as found in the CPAN module List::Pairwise:

use List::Pairwise qw(grepp mapp);
my %k = (a=>1, b=>2, c=>undef);
say for mapp { $a } grepp { !$b } %k;

Answer 2

Yes:

$b or say $a while ($a,$b) = each %k

But that's not any better (worse, IMO), so I'd stick with what you have.