perl-opengl画距形

#!/usr/bin/perl -w
use strict;
use warnings;
use OpenGL qw/ :all /;
use OpenGL::Config;   

glutInit();
glutInitDisplayMode(GLUT_RGB|GLUT_SINGLE);
glutInitWindowPosition(100,100);
glutInitWindowSize(400,400);
glutCreateWindow("my  OpenGL program");
glClearColor(0,0,0,255);
glClear(GL_COLOR_BUFFER_BIT);  
glMatrixMode(GL_PROJECTION);
gluOrtho2D(-100,100,-100,100);
glutDisplayFunc(\&mydis);
glutMainLoop();
return 0;

sub mydis()
{
  glColor3f(0,255,0);
  glBegin(GL_LINES);
  glVertex2f(-100,0);
  glVertex2f(100,0); 
  glEnd();
  glBegin(GL_LINES);
  glVertex2f(0,-100);
  glVertex2f(0,100); 
  glEnd();  
  glColor3f(255,255,0);  
  glRecti(-50,-50,50,50);
  glFlush();
}