my ($sum) = $sth->fetchrow_array;
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Or, if you have your SQL in a variable...

my ($sum) = $dbh->selectrow_array( $sql, undef, $foo, $bar, $rat );
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DBI has all kinds of good stuff.

Update: When there is no result, $sum will be undef. You can do $sum ||= 0 (or $sum //= 0 in Perl 5.10) to avoid generating warnings about this later.