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use Array::Utils qw( intersect );
my @refkeys1 = grep { ref $ref1->{$_} } keys %$ref1;
my @refkeys2 = grep { ref $ref2->{$_} } keys %$ref2;
my @common = intersect( @refkeys1, @refkeys2 );

use strict;
use warnings;
...

my $output = mergehash($ref1,$ref2);
print Dumper $output;