Any number of sets can be passed (not just two or three).
By BK 

my @numericUnion = union{ $_[0] == $_[1] } [1,2,3], [2,3,4], [9]; #ret
+urns ( 1, 2, 3, 4, 9 ) 

my @stringUnion = union{ $_[0] eq $_[1] } [ qw( aaa bbb ccc ) ], [ qw(
+ bbb ccc ddd ) ]; #returns( 'aaa','bbb','ccc','ddd' ) 

 my @unique = union{ $_[0]->isSameNode } [ qw( list of xml nodes ) ], 
+[ qw( othrer list of xml nodes ) ] #unique node list


sub union(&@){
    my $eq = shift;
    @_ == 1 ? @{ $_[0] } :
    union(
        $eq, [
            @{ $_[0] },
            grep &{sub{
                !grep $eq->( $_, $_[0] ), @_[1..$#_]
            }}( $_, @{ $_[0] } ), @{ $_[1] }
        ], @_[2..$#_]
    )
}

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In reply to Let's unite sets by songahji

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