I was working on a GD based module, when i came up with a functional fix (so, it's not pretty, but mostly just to prove it works).

#!/usr/bin/perl

use GD;

my $image = new GD::Image(110,110);

my $red = $image->colorAllocate(255,0,0);      
my $blue = $image->colorAllocate(0,0,255);
my $white = $image->colorAllocate(255,255,255);
$image->fill(0,0,$white);

my $poly = new GD::Polygon;
$poly->addPt(50,0);   ### add your points
$poly->addPt(99,99);
$poly->addPt(0,99);
$image->polygon($poly);

### do the work
my $base_point = [109,109]; # known outside
my $test_point = [50,50];   # unknown

$image->fill($test_point->[0],$test_point->[1],$red);

my $clrindex = $image->getPixel($base_point->[0],$base_point->[1]);
if ($clrindex == $red) {
    warn "Point is outside of polygon.";
    exit 0;
} else {
    warn "Point is inside of polygon.";
    exit 1;
}
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so, if it is the answer you seek, i like this way ... if it is the mathmatics of it (which it probably is), this is not use.

---

_{can't sleep clowns will eat me}
-- MZSanford