I use the following code (using FrameBufferObjects) to save screenshots. I render into texture $texture_id and then read that texture back into main memory:

sub tick {
    my ($self,$texture_id) = @_;

    # save current frame from input fbo
    glBindTexture(GL_TEXTURE_2D, $texture_id);
    my $frame;
    for my $i (reverse (0..$self->height-1)) {
        glReadPixels_s(0,$i,$self->width,1,GL_RGB,GL_UNSIGNED_BYTE,my 
+$line);
        $frame .= $line;
    };
    syswrite $self->stream, $frame, $self->width * $self->height * $se
+lf->depth
        or die "Write failure";
};
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... and here's the part that renders into an fbo:

sub allocate_fbo_texture{
    my ($class,$width,$height) = @_;
    (my $texture_id) = glGenTextures_p(1);
    (my $fbo) = glGenFramebuffersEXT_p(1);
    (my $render_buffer) = glGenRenderbuffersEXT_p(1);
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, $fbo);
    glBindTexture(GL_TEXTURE_2D, $texture_id);
    glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, $render_buffer);

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexImage2D_c(GL_TEXTURE_2D,0,GL_RGBA8,$width,$height,0,GL_RGBA,G
+L_UNSIGNED_BYTE,0);

    glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0
+_EXT, GL_TEXTURE_2D, $texture_id, 0);
    glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT24
+, $width,$height);
    glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHME
+NT_EXT, GL_RENDERBUFFER_EXT, $render_buffer);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    return ($fbo,$texture_id, $render_buffer);
};

sub enable {
    my $self = shift;

    glPushAttrib(GL_VIEWPORT_BIT);
    glViewport(0,0,$self->width, $self->height);
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, $self->fbo);

    my $res = $self->shader->Enable();

    if ($self->config->{enable}) {
        $self->config->{enable}->($self);
    };
};
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This is quite roughly ripped out of a larger framework, but it should give you at least a hint of how you could do it. I'm not sure why your approach of reading the memory doesn't work though.

#!/usr/bin/perl
use warnings;
use strict;
use Image::Magick;
use X11::WMCtrl;
use Data::Dumper;

my $wmctrl = X11::WMCtrl->new;

printf("window manager is %s\n", $wmctrl->get_window_manager->{name});

my @windows = $wmctrl->get_windows;

my $wid;  # x window id
my $host; # virtual desktop

foreach my $app(@windows){
         my $title = $app->{title};
          if( $title =~ /Mozilla/i){
          print "$title\n";
          print Dumper(\$app),"\n\n";
          
          $wid = $app->{id};
          $host = $app->{host};

         }
   }

$wmctrl->switch($host);
#must be on same virtual desktop as the browser

#works
my $blob = `import -window $wid jpg:`;   

#my $blob = `import -`;  #postscript produced
#print "$blob\n";
# now $blob is in memory and you can do what you
# want with it.

my $output = Image::Magick->new(magick=>'jpg');
$output->BlobToImage( $blob );
$output->Resize(geometry=>'160x120');
$output->Write( $0.'.jpg');

exit;
# png works too
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well i tried both solutions thank you very much ... but i keep on getting a black screen. I tried image magick bevor. I am using activestate perl under windows and i would be really happy if anybody has any solution to create screenshots of full screen directx games / applications. I tried many ways but i keep on getting black screens thanks

Err - DirectX is quite different from OpenGL.

ilInit();
iluInit();
...
ILuint ratz;
ilGenImages(1, &ratz);
ilBindImage(ratz);
vector<GLubyte> store;
store.resize(Wide * Height * 4);
glReadPixels(0, 0, Wide, Height, GL_RGBA, GL_UNSIGNED_BYTE, &store[0])
+;
ilTexImage(Wide, Height, 1, 4, IL_RGBA, IL_UNSIGNED_BYTE, &store[0]);
ilEnable(IL_FILE_OVERWRITE);
ilSaveImage(sortie.c_str());
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