my @results;
while (my @row = $sth->fetchrow_array) {
   push @results,\@row;
}
if (@results) {
   ...we got something...
} else {
   ...we got nothing...
}

##</code><code>##

my $results = 0;
while (my @row = $sth->fetchrow_array) {
   $results++;
   ...do some stuff
}
if ($results) {
   ...we got something...
} else {
   ...we got nothing...
}