scalar + selectall_array (or selectall_arrayref might be your friends here). If you need a memory "lite" option and can afford an second SQL call, there's always the SQL COUNT(*) dealio.
But if say you have all rows in $results_ref (an array ref); it'd be like:

my $results_ref = $dbh->selectall_arrayref(....);
if (@$results_ref) {
  # ... do stuff
}

# or just get $count, (using `scalar` for clarity but it's already in 
+scalar
# context so works without it here)
my $count = scalar @$results_ref;
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