As 0 is most likely an invalid/undefined birthday too, you just want "true" values. All good advice already given, the shortest route might be:

printf "%-12s %s\n", $_, $birthdays{$_} for grep { $birthdays{$_} } so
+rt keys %birthdays;
[download]

-->

$ perl test.pl

Name         Birthday
-------------------------
Bob          August 28
Fred         June 1
Glenn        May 27
Ray          May 31
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