Looking at the source code of Chart::Gnuplot, I see that it calls the program as:

    my $err = `$cmd 2>&1`;
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So basically, it will always wait until $cmd finishes. You can try to reconfigure/replace $cmd by a command that launches Gnuplot in the background and then immediately returns. This would return control to your program, but will always launch a new instance of the gnuplot program for each diagram you want to display I imagine.

I fear the only way to integrate things more into your program is to make gnuplot return image data and to render that yourself. But maybe by reading the documentation or the source code you find a way that I missed.