DELIMITER //
DROP PROCEDURE IF EXISTS DebugMe //
CREATE PROCEDURE DebugMe(OUT arg char(10))
BEGIN
    SET arg = "HELLO";
END //
DELIMITER ;
CALL DebugMe(@blah);
SELECT @blah;

Query OK, 0 rows affected (0.00 sec)

+-------+
| @blah |
+-------+
| HELLO |
+-------+
1 row in set (0.00 sec)

##</code><code>##

use warnings;
use strict;
use DBI;

my $dsn = 'DBI:mysql:database=test';
my $dbh = DBI->connect($dsn, @ARGV, {RaiseError => 1});
my $sth = $dbh->prepare('SELECT "Hello World"');
$sth->execute();
printf "SELECT rows = %d\n", scalar $sth->rows;
printf "\t(@$_)\n" for $sth->fetchrow_arrayref;

my $v;
$sth = $dbh->prepare('CALL DebugMe(?)');
$sth->execute($v);
$sth->execute(\$v);
__END__
SELECT rows = 1
        (Hello World)
DBD::mysql::st execute failed: OUT or INOUT argument 1 for routine test.DebugMe is not a variable or NEW pseudo-variable in BEFORE trigger at sscce.pl line 14.
DBD::mysql::st execute failed: OUT or INOUT argument 1 for routine test.DebugMe is not a variable or NEW pseudo-variable in BEFORE trigger at sscce.pl line 14.