I used a stored procedure for encapsulating that behaviour when I had a similar problem in the past, but if you don't have permission to add stored procedures you should be able to run the sequence of commands inline. I don't have access to a server to test with currently though, so your mileage may vary.

Something like this:

my $sql = 'DECLARE @ptrval binary(16);'
        . ' SELECT @ptrval = TEXTPTR(item)'
        . ' FROM table '
        . ' WHERE whatever criteria;'
        . ' UPDATETEXT table.item @ptrval NULL 0 ?';
$dbh->do($sql,{},"new text");
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But of course that's horribly ugly.

So, a stored procedure to encapsulate the behaviour:

CREATE PROCEDURE dbo.APPEND_THINGY
@id int,
@new_text varchar(8000)
AS

DECLARE @ptrval binary(16)
SELECT @ptrval = TEXTPTR(item) 
   FROM table 
   WHERE id = @id

UPDATETEXT table.item @ptrval NULL 0 @new_text;

GO
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And then in your code:

my $sql = q{exec append_thingy @new_text = ?};
$dbh->do($sql,{},"new text");
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