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my %seen;
my @array = grep !$seen{$_}++, LIST;

my %seen;
my @array = grep !$seen{$_}++, @array, $new_value;

my %edges;
while (<STREAM>) {
...
   my %seen;
   @{$edges{$node1}} = grep !$seen{$_}++, @{$edges{$node1}}, $node2;
}

my %edges;
my %seen;
...
   next if !$seen{$node1}{$node2}++;
   push @{$edges{$node1}}, $node2;
}