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my @array1 = (1, 3, 4, 6);
my @array2 = (1, 2, 4, 6);
...
@union = grep { not $seen{$_}++ } (@union, @array3);

print join(' ', @union);

@array1 = (1, 2, 3, 4);
@array2 = (1, 3, 2, 4);