What is the problem ?

Allow me to introduce you to splain, a command-line interface to perldiag.

C:\>splain
C:\Perl\bin/splain.bat: Reading from STDIN
Attempt to bless into a reference
Attempt to bless into a reference (#1)
    (F) The CLASSNAME argument to the bless() operator is expected to 
+be
    the name of the package to bless the resulting object into. You've
    supplied instead a reference to something: perhaps you wrote

        bless $self, $proto;

    when you intended

        bless $self, ref($proto) || $proto;

    If you actually want to bless into the stringified version
    of the reference supplied, you need to stringify it yourself, for
    example by:

        bless $self, "$proto";
[download]

So it turns out there is actually a bug in the OOXML.pm file.
In subs new, tag, and text, he calls $self->new(...) which is precisely the trigger for the diagnostic above.
Probably the easiest way to fix this is just to change return bless $self, $class; into return bless $self;
That's not robust if OOXML is going to be subclassed, however. In that case, I'd recommend the following simple change instead:

In sub new, immediately after the my( $class, ... line, insert the following line:

    $class = ref($class) if ref($class);
[download]