@c = map { "$a[$_]-$b[$_]" } (0..(($#a > $#b) ? $#a : $#b));

# And a more twisty way...

my $iterator;
{
    my $i = 0;
    $iterator = sub {
        ($i = 0, return undef) if (@_);
        return undef if ($i > $#a and $i > $#b);
        return ("$a[$i]-$b[$i]",$i++)[0];
    };
}
push @c, $_ while ($_ = $iterator->());