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.