MUMmer plot error. Line 884. What to do?

My third year comp sci friend got me this far, I hope you guys can help me with the last inch! For the record, I have practically no coding experience. I am using the 3.9.4 alpha version of MUMmer.

Here is the manual I have been following: http://mummer.sourceforge.net/examples/

I am at part 2.2.2 Running the Mummer plot, and I type in the final line to generate the plot.

I type:

mummerplot -x "[0,8666124]" -y "[0,9025688]" -postscript -p mummer mummer.mums

And the error is:

Can't use 'defined(%hash)' (Maybe you should omit the defined ()?) at /usr/bin/mummerplot line 884

Line at 884:

if ( defined (%$rref) ) {                                                                                                            #-- skip reference sequence or set atts from hash
        if ( !exists ($rref->{$idR}) ) { next; }
        else { ($refoff, $reflen, $refdir) = @{$rref->{$idR}}; }

Its perl