RTFM. man DBI, and early on, in the outline usage section, you find:

       The typical method call sequence for a "SELECT" statement
       is:

         prepare,
           execute, fetch, fetch, ...
           execute, fetch, fetch, ...
           execute, fetch, fetch, ...
[download]

And further down, when discussing the do method:

                                   It should not be used for
           "SELECT" statements because it does not return a
           statement handle (so you can't fetch any data).
[download]

Consider consulting the manual before asking questions like this.

Abigail