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@union = sort grep { not $seen{$_}++ } (@union, @array3);

my @array1 = ([b=>1], [d=>3], [z=>4], [e=>5]);
my @array2 = ([a=>2], [b=>1], [z=>4]);
my @array3 = ([d=>3], [e=>5]);

my %seen;
my @union = grep { not $seen{$_->[0]}++ } (@array1, @array2);
...

use Data::Dumper;
print Dumper \@union;