For the immediate problem,

sub int2mon {
    (Jan=>Feb=>Mar=>Apr=>May=>Jun=>Jul=>Aug=>Sep=>Oct=>Nov=>Dec=>)[shi
+ft]
}
[download]

will do the job, but you should be working with one of the Date modules. That link will list several.

The code above just indexes an anonymous array of strings with the first argument given the sub.

After Compline,
Zaxo