Thank you for your comment, AnomalousMonk. I thought that something like:

would return a count on only the defined values. After a quick try, this is obviously not the case. My error.

This leads me to another possible solution illustrated in the following session under the Perl debugger:

  DB<1> %hash = (jan => 1, feb => 2, mar => undef, apr => undef, may =
+> 5);

  DB<2> x %hash
0  'feb'
1  2
2  'jan'
3  1
4  'may'
5  5
6  'mar'
7  undef
8  'apr'
9  undef

  DB<3> print scalar grep {defined $_} values %hash;
3
  DB<4> print scalar grep {defined $_} keys %hash;
5
[download]