By default, the key sequence for language-sensitive text selection in
  the source view is Ctrl/MB1.  To define another key sequence, edit the
  current definition of the following resource in the VMSDEBUG.DAT
  resource file:

  DebugSource.IdentifierSelectionSequence

  To avoid conflict with standard DECwindows Motif word selection, do not
  use a double-click combination, such as Ctrl<Btn1Down>(2).