When you grant permissions to this script a button will appear on screen that if pressed will send your view into mouselook. The script will (when in mouselook) react to your left mouse button clicks too.

// V1 //
 
key owner;
 
default
{
    state_entry()
    {
        owner = llGetOwner();
        llRequestPermissions(owner, PERMISSION_TAKE_CONTROLS);
    }
    run_time_permissions(integer perm)
    {
        if(perm & PERMISSION_TAKE_CONTROLS) // Do we have the perms we asked for?
        llTakeControls(CONTROL_ML_LBUTTON, TRUE, FALSE); // This creates a button at the bottom of the screen
        else                    // that if pressed automatically zooms the camera of the owner into mouselook.
        llRequestPermissions(owner, PERMISSION_TAKE_CONTROLS);
    }
    control(key id, integer this, integer that)
    {                        // Adding various conditions here can change the script behavior in many ways.
        llOwnerSay("Click"); // Do stuff when clicking the left mouse button while in mouselook. Like for guns etc.
    }
}