my $sql = "SELECT COUNT(*) FROM Person WHERE Account_idAccount = 35 AND JSON_EXISTS(custom, '$.test3')");
say "Running <<$sql>>";
my $res = $dbh->selectrow_array($sql);
...

##</code><code>##

my $sql = <<'SQL';
    SELECT COUNT(*) FROM Person WHERE Account_idAccount = 35 AND JSON_EXISTS(custom, '$.test3')
SQL
...