First of all, it would've been more appropriate for you to respond to the original post, not to ikegami's reply — he didn't use map the wrong way :)

Next, while your observation is correct, you seem to have overlooked the fact that the original poster is getting an unwanted trailing comma.

Your solution could be fixed by changing the output field separator $,

...
$, = ",";
print @foo;
[download]

or (to keep the effect localized)

...
{
    local $, = ",";
    print @foo;
}
[download]