but the "map" is not doing what i expected

That's because the result of the map that is passed out is the result of the last statement in it. You are not passing $_ out of your map, just the result (number of substitutions) of the s/ $//;. Consider the following two code snippets.

$ perl -E '
> @arr = qw{
>    abCdeFg
>    hIJklMn
>    OpqrsTu
>    };
> say join q{:}, @arr;
> $str = join q{:},
>    map { s{[A-Z]}{*}g } @arr;
> say $str;'
abCdeFg:hIJklMn:OpqrsTu
2:3:2
$
[download]

$ perl -E '
> @arr = qw{
>    abCdeFg
>    hIJklMn
>    OpqrsTu
>    };
> say join q{:}, @arr;
> $str = join q{:},
>    map { s{[A-Z]}{*}g; $_ } @arr;
> say $str;'
abCdeFg:hIJklMn:OpqrsTu
ab*de*g:h**kl*n:*pqrs*u
$
[download]

Notice the different result when I pass $_ out of the map by mentioning it in a final statement.

I hope this is helpful.