The execute() method called from your statement handle returns the number of affected rows, as does $sth->{'mysql_affected_rows'}.

*NOTE* I'm think this is mysql specific for selects

You probably want something like:

my $sth = $dbh->prepare(
  "SELECT * FROM poll 
   WHERE status='waiting' 
   ORDER BY `id` ASC 
   LIMIT 0, 5"
  );
my $query_count = $sth->execute or die $dbh->errstr;
if ( $query_count == 1 ) {
  # you *can* pull one but just one row using one of the 
  # fetch* methods
} else {
  # your condition isn't met
}
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