selectall_arrayref returns a reference to an array of rows.

my $all = $dbh->selectall_arrayref("
    SELECT ID, col1, col2
      FROM table
     WHERE col1 = 'xxx'
");

print "Number of rows selected: ", 0+@$all, "\n";
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or if you don't actually need the rows:

my ($count) = $dbh->selectrow_array("
    SELECT COUNT(*)
      FROM table
     WHERE col1 = 'xxx'
");

print "Number of rows selected: $count\n";
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