You should not ->finish () select handles that are done reading:

my $sth = $dbh->prepare ("select * from foo");
$sth->execute;
while (my $ref = $sth->fetch) {
    print $ref->[0], "\n";
    }
# No need for $sth->finish;
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The safest way to never worry about this is to make those select loops have the smallest possible context, so the handle gets DESTROY'd on leaving scope:

{   my $sth = $dbh->prepare (...);
    $sth->execute;
    ...
    } # End-of-scope: $sth is destroyed
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It is usually advisable to finish update/insert/delete handles, as they have no obvious "done" state.

{   my $stu = $dbh->prepare ("update foo set blah = ? where c_foo = ?"
+);
    my $sth = $dbh->prepare ("select c_foo, bar from brimble");
    $sth->execute;
    while (my $r = $sth->fetchrow_hashref) {
        $stu->execute ($r->{bar}, $r->{c_foo});
        }
    # no need for $sth->finish
    $stu->finish; # Good practice
    } # End of scope: clean up
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