use Data::Dumper;

my $hash1 = {
	common_key => 'foo',
	hash1_specific_key => 'some value'
};

my $hash2 = {
	common_key => 'bar',
	hash2_specific_key => 'some other value'
};

my $joined_keys = [ keys( %{ {
	map { $_=>undef }  keys( %{$hash1} ) , keys( %{$hash2} ) 
} } ) ];

print Data::Dumper->Dump([$joined_keys],['joined keys']);