Good, then we don't even require conditionals. Simply make two passes over the arrays: the first to remove elements that exist in @in; the second to append elements in @in to @db:

#my @db = (1, 3, 5, 7, 9, 11);
#my @in = (1, 2, 5, 8, 9, 10, 13);
my @db = (1, 3, 5, 7, 9, 11);
my @in = (21, 22, 25, 28, 29, 210, 213);

my (%db_lookup, %in_lookup);
@db_lookup{@db} = ();
@in_lookup{@in} = ();

my @out = grep { not exists $in_lookup{$_} } @db;
push @out, grep { not exists $db_lookup{$_} } @in;

print "view: @out \n";
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Output from the first set is: 3 7 11 2 8 10 13. This differs from your sample output, but I think this actually does what you want. The second sample gives: 1 3 5 7 9 11 21 22 25 28 29 210 213, which matches exactly.