Doing everything in the database using SQL statements — as suggested by others here — is a very good option if you are just working within the database itself (i.e. inserting, updating).

However, you posted your question to a Perl forum, not a SQL forum, so I will assume that you need to do something with the data in Perl. As such, you actually need to fetch table data (rather than table metadata such as that returned by commands such as $sth->rows). To do this, you need to make sure that your statement handle is correctly formulated to return the data you want, and then you will use some version of fetchrow:

my $key_count;
while ( my ( $keys ) = $sth->fetchrow_array ) {
    $key_count += $keys;
}
# Do something with the total $key_count
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Hopefully, this is enough to point you in the right direction.