Dear list'ers

I'm struggling with a surface shade plot and can't figure out what I'm doing wrong. I can't find any info other than the plplot homepage examples regarding PerlDL, and all I get by following those examples are just black surfaces...

Anyway, here's my problem (and it should be simple to solve, argh!!!):

I've got three different arrays, $lon, $lat and $value.
$value[0] corresponds to $lon[0] and $lat[0] and so on.
$value takes values from 0.0 to about 20.0.
The lon/lat grid resolution is 2.5 X 2.5 deg and may take any geographical value (-180 to180 and -90 to 90)

I'd like to make a surface shade plot (blue -> green -> red) showing red for the highest values.

Sorry to bother you with such basic questions...

Cheers,
Ked