I'm not sure what you're trying, is that perl!
Have a look at the Mysql manual (http://dev.mysql.com/doc/refman/5.0/en/insert-select.html)
Here is an attempt of what I think you're trying

my $sql = "INSERT INTO $db.$table (category) SELECT $oldtable.category
+ FROM $oldtable" ;
[download]

If this isn't what you're looking for! please ignore me :-)

Cheers
LuCa