$header_id contains a hash reference (in this case it is a blessed reference, indicating that you have created a DBI object), not your data. This is good, DBI is doing exactly what we want it to when we call execute(). It is creating a set handler to provide us with a few different ways of accessing the results of our query-- probably the most useful access method here is line-by-line as a list.

Your code should probably look more like:

$sth = $dbh->prepare( $SQL );
$sth->execute();
@header_ID = $sth->fetchrow_array();
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Now $header_ID[0] will work more how you expected. For more information 'perldoc DBI' or this: Reading from a database.