Thanks to everyone who took the time to respond.

I think the documentation may be slightly off. Here's what appears in the documentation I was referred to:
...
$sth = $dbh->prepare("declare \@id_value int, \@id_name exec my_proc @name = 'a string', @number = 1234, @id = @id_value OUTPUT, @out_name = @id_name OUTPUT");
...

Question - isn't the backslash (\) character needed in front of every '@' character within the string containing the stored procedure call ?