Might also try inlining Determinant and some common sub-expression elimination...
sub SegmentIntersection {
my @points = @{$_[0]};
my @p1 = @{$points[0]}; # p1,p2 = segment 1
my @p2 = @{$points[1]};
my @p3 = @{$points[2]}; # p3,p4 = segment 2
my @p4 = @{$points[3]};
my $a = $p2[0] - $p1[0];
my $b = $p3[1] - $p4[1];
my $c = $p2[1] - $p1[1];
my $d = $p3[0] - $p4[0];
my $det = $a*$b - $c*$d;
return 0 if (abs($det) < $delta); # parallel
my $e = $p3[1]-$p1[1];
my $f = $p3[0]-$p1[0];
my $n1 = $f*$b - $e*$d
my $n2 = $a*$e - $c*$f;
if ($det > 0)
{
return $n1 < $det && $n2 < $det && $n1 > 0 && $n2 > 0;
}
else
{
return $n1 > $det && $n2 > $det && $n1 < 0 && $n2 < 0;
}
}