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

##</code><code>##

...

use Data::Dumper;
exists $mon->{JAN}
    or die "Couldn't find entry for 'JAN' in " . Dumper $mon;