MUMmer plot error. How to resolve?

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: 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.

[email protected]:/mnt/c/mummer/MUMmer-3.9.4alpha$ mummerplot -x "[0,8666124]" -y "[0,9025608]" -postscript -p mummer mummer.mums 
Can't use 'defined(%hash)' (Maybe you should just omit the defined()?) at /usr/bin/mummerplot line 884.

So I go to investigate the error and I dont know how to proceed from here (this is the line I get once I ctrl x dash and enter 884).

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

What should I do?