It would appear that the memory space on which the string is stored is not modifiable. But that's not expert opinion, rather just from empirical testing. Can't memset the location, so therefore it's not modifiable. And strtok modifies the string you pass it.
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...