If you had used warnings or ran your script as perl -w 1041970.pl, Perl would've given you a hint on what goes wrong:

Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
Use of uninitialized value in numeric comparison (<=>) at tmp.pl line 
+10.
APR12 MAR13 APR11 MAR12 FEB13 APR13
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Now, the task is to find out why $mon->{$a} might be undef. You could for example do a first simple sanity check:

...

use Data::Dumper;
exists $mon->{JAN}
    or die "Couldn't find entry for 'JAN' in " . Dumper $mon;
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That should give you a hint on how to proceed.