In this particular code, comp_includes_z doesn't have any function. All that does is indicate that when comparing two points you want to exclude the Z value from the comparison. For example:

use Geo::ShapeFile::Point comp_includes_z => 0;

my $p1 = Geo::ShapeFile::Point->new(X => 3, Y => 5, Z => 10);
my $p2 = Geo::ShapeFile::Point->new(X => 3, Y => 5, Z => 25);

# this will return false if comp_includes_z is true,
# and return true if comp_includes_z is false
if($p1 == $p2) {
  print "these points are the same spot\n";
}
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