Your version of perl probably implements
qw(A B C)
as
split(' ', q(A B C))
(although I suspect it performs said split at compile-time).

Given the above, consider the following passage from split's documentation:

In scalar context, returns the number of fields found and splits into the @_ array. Use of split in scalar context is deprecated, however, because it clobbers your subroutine arguments.

If you want the count, use my $scalar = () = qw(A B C);.

By the way, I said "your version of perl" because perl 5.6.1 gives

Useless use of a constant in void context at 541192.pl line 4.
Useless use of a constant in void context at 541192.pl line 4.
C
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