You have to single quote all the month numbers which start with a 0 or otherwise Perl thinks you are using octal numbering (and as there is no such thing as an octal digit 8 or 9 you get the "illegal octal" error).

To get the long month name out of your %months hash, just index this hash with the key: $months{$outputdates[0]}. It's like ordinary arrays (which are indexed by a number, but you use the key value here).