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 use Imager;
 # OPEN FILE
 my $fn = 'datafile.dat';
 open my $fh, '<', $fn or die "$fn $!";

 # READ DATA INTO 2D MATRIX
 my @matrix = map [ qw(-1) x 40 ], 1..40;
 $matrix[$_->[1]][$_->[0]] = $_->[2] for map [split /\s+/ ], <$fh>;
 
 # PREPARE IMAGE
 my $scale=10;
 my $img = Imager->new(xsize=>40*$scale, ysize=>40*$scale, channels=>3);
 $img->box(filled=>1, color=>'#FFFFFF');

 # CREATE IMAGE
 for my $row (0 .. @matrix-1) {
    for my $col (0 .. @{$matrix[$row]}-1) {
       my $val = $matrix[$row][$col];
       next if $val == -1;
       # SELECT APPROPRIATE COLOR (BE CREATIVE HERE ;-)
       $img->box( color=>[ ($val/10)*25, $val*2.5, $val ],
                  filled=>1,
                  xmin=>$col*$scale,     ymin=>$row*$scale,
                  xmax=>($col+1)*$scale, ymax=>($row+1)*$scale )
    }
 }
 # DUMP IMAGE TO FILE
 $img->write(file=>'my.jpg', jpegquality=>90) or die $img->errstr;
 ...