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my @key_array = 'a' .. 'z';
my @set_array = ( ['a', 'b'],  ['e','f'], ['f','g'] );

use strict; use warnings; use Data::Dumper;
my @key_array = 'a' .. 'z';
...
for ( grep {$key_hash{$_} == 0 } sort {$a cmp $b} keys %key_hash ){
    print "$_\n";
}