It may be that your vendor library isn't too clean with its memory management. The order of object creation and object destruction differs whether you undef the first object manually or wait until it gets overwritten:

sub new{
    print 'new';
    bless{}
};

sub DESTROY{
    print'DESTROY'
};

$x=new;
$x=new;
__END__
new
new
DESTROY
DESTROY
[download]

and with explicit undef:

sub new{
    print'new';
    bless{}
};
sub DESTROY{print'DESTROY'};
$x=new;
undef $x;
$x=new
__END__
new
DESTROY
new
DESTROY
[download]

at least under Perl 5.10, but I don't see how it could be different under any Perl ≤ 5 due to the fact that the assignment will always happen after the object construction, and only after the assignment, the memory allocated for the first instance can be released.

Update: Doh - had I read your node to the end, I'd have seen that you came to that conclusion already...