That's not exactly what I would call "joining two arrays"; you've actually created a List of Lists. Joining arrays is much simpler than that:

my @a = 1 .. 4; my @b = "a" .. "d";
my @c = (@a, @b);

# You could even do this - no need to create a new variable:
@a = (@a, @b);

# Or...
push @a, @b;
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There's also no need for the complex and hard-to-read map statement if you wanted to create a LoL; just as in your example, this assumes that the first array is the same length or shorter than the second one:

my @c;
push @c, [ $a[$_], $b[$_] ] for 0 .. $#a;
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Update: Added the "Or..." "push" method.