Are you using Oracle? Oracle does some tricks behind the scenes to make things work as if you had used bind variables even when you don't. It's not as good as real bind variables in all cases, but it might account for what you're seeing here. (This is discussed in the latest version of Tim Bunce's DBI talk which you can find on CPAN.)

If you really want to speed things up, change that fetchrow_array() to fetchrow_arrayref() or use bind_columns().