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%a = ( one=>1, two=>2, three=> 3 );
%b = ( two=>'two', three=>'three', four=>'four');

if (exists $a{one}) { print "\%a has 'one' for a key" }

print join(",", keys %a); # prints "one,two,three"

my @common_keys;
foreach my $key (keys %a) {
...
      push @common_keys, $key;
   }
}

my @common_keys = grep { exists $b{$_} } keys(%a);

$x = \%a;
$y = \%b;

my @common_keys = grep { exists $y->{$_} } keys( %{ $x } );