my @combined = map {"$a1[$_]:$a2[$_]"} ($#a1 < $#a2 ? (0..$#a2) : (0..$#a1));

##</code><code>##

my @combined = map {($a1[$_]//"").":".($a2[$_]//"")} ($#a1 < $#a2 ? (0..$#a2) : (0..$#a1));