If your requirement is simple, an easy alternative might be create your own version of the module with this line added

        else {
            # draw main bar face
            my $centretopy = $ypos + ($dims{plotheight} - (($dims{plot
+height}/$dims{range})*$d->[1])) + $floordepth;
            $image->filledRectangle($barpos,$centretopy,$barpos+$conf{
+bwidth},$ypos+$dims{plotheight}+$floordepth,$colbar);
### add this line
      $image->string(gdSmallFont,$barpos,$centretopy-20,$d->[1],$colba
+r);
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