Well, scalar enforces scalar context, not list context. It's the LHS of the assignment that determines context. Putting the readdir in list context is easy:

    my ($entries) = readdir $dh;
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Note that this not solve your problem. Because you phrased it as an XY problem. You want to solve X, you think Y solves X, so you ask how to solve Y. But Y (putting readdir in list context) does not solve X (finding out the number of entries in a directory). At least, not directly. You can assign the result of readdir to a list and put that assignment in scalar context; this will give you the number of entries in the directory. One way of doing this is:

    my $entries = () = readdir $dh;
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The parens here indicate an (empty) list.