You should not use Input inside the OnGUI function.
Also GetKeyDown, only returns true when the key is first pressed. GetKey returns true as long as the key is pressed.
Instead set a flag and use that inside OnGUI.
var showingTextArea = false;
function Update ()
{
showingTextArea = Input.GetKey (KeyCode.LeftControl);
}
function OnGUI ()
{
if(showingTextArea) {
GUI.TextArea( textrect, formText );
}
}