Obviously you need to declare your variables (using my).

I think you could stand to improve your code formatting. try this on:

use strict;
use warnings;


glpOpenWindow;
glClearColor(0,0,0,1);
glClear(GL_COLOR_BUFFER_BIT);
glOrtho(-3,3,-3,3,-3,3);
glColor3f(1,0,0);

my $loops = 6;
my $radius = 2.5;

sub rect($$$$$$)
{
    my( $ulx, $uly, $lrx, $lry, $rot, $green ) = @_;

    glColor3f( $rot/(180*$loops)/2, $green, abs(100-($rot/360))/2 );
    # the following must come after the preceding.
    $rot = defined($rot) ? int($rot) * 0.017453 : 0;

    my $ofsx = cos($rot)/2.5;
    my $ofsy = sin($rot)/2.5;
    glBegin(GL_POLYGON);
    glVertex2f( $ulx-$ofsx, $uly+$ofsy );
    glVertex2f( $ulx-$ofsx, $lry+$ofsy );
    glVertex2f( $lrx+$ofsx, $lry-$ofsy );
    glVertex2f( $lrx+$ofsx, $uly-$ofsy );
    glEnd();
    glpFlush();
}

for ( my $i = 0; $i<180*$loops; $i++ )
{
    $radius /= 1.00061;
    my $yoff = cos($i/157);
    rect( -1*$radius, -1*$yoff*1.5, 1*$radius, 1*$yoff*1.5, $i, 0 );
}
rect( -2.5 ,  -1  , 2.5 , 1    , 0, 0.2 );
rect( -2.47, -0.97, 2.47, 0.97 , 0, 0   );
rect( -2.4 , -0.94, 2.43, 0.943, 0, 0.2 );
glpMainLoop;
[download]