See Example 7 in the docs. All you need to do is define xtics with the labelfmt of your choice. Something like the following (untetsted, I don't have Chart::Gnuplot on this machine):

my $chart = Chart::Gnuplot->new(
        output => "/home/hmadhi/workspace/CommsNet/www/Plots/$kpi.png"
+,
        title  => {text=>"$kpi Success Rate on $node/$ran ($interface)
+ $date",
                  },
        xlabel => {text=>"Time",},
        ylabel => "$kpi Succ_Rate",
        timeaxis => "x",
        xtics    => {
                       labelfmt => '%H:%M',
                    },

    );
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