It would have been useful if you had provided some detailed information as to how your code failed, such as error or warning messages or a small sample of your output.

Regardless, I see a few problems, in addition to the sort issue mentioned by others. You would have gotten warning messages had you used the strictures:

use warnings;
use strict;
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You would have been warned about month_from being a bareword and the Useless use of sort in void context. Also, use diagnostics can be used to get more verbose warning messages.

Another problem is that you should always check if a regex match succeeded before using $1, $2, etc.