Assuming I've not misintepreted your question, I'd lean towards a solution something like this:

my @defaults = qw/one two three four five/;
my @a = qw/un deux trois/;

$a[$_] ||= $defaults[$_] for (0 .. $#defaults);

print "@a\n";
# prints "un deux trois four five"
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Again, if you're looking for a completely either-or approach to the two lists, then the aforementioned trinary operator is probably the best choice. If you want to take from @a or @defaults on a per-element basis, then the trinary operator won't give you the desired results -- you'll need to use an approach like mine or RMGir's above.

blokhead