/sys$common/syshlp/helplib.hlb PIPE, Description, DCL Command Restrictions  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The PIPE command creates a special execution context for its
    command sequences. The following DCL commands either do not work
    or exhibit new behavior in this context:

    o  PIPE - Nested PIPE commands in the same command procedure
       level are not allowed. There can only be one PIPE command
       context for each command procedure level; however, nested
       PIPE commands at different procedure levels are allowed. For
       example:

        $ TYPE FOO.COM
        $ ! FOO.COM
        $   :
        $ PIPE   ...
        $ :
        $
        $ PIPE    @FOO.COM ; ...

       In this example, the PIPE command inside FOO.COM is allowed
       because it is executed at a different command procedure level.

    o  GOTO and EXIT - These two commands, when executed as PIPE
       command sequences, delete the PIPE command context before
       the GOTO or EXIT command is executed. Any command sequences
       following these two commands in a PIPE command are flushed.

    o  STOP - The STOP command, when executed after a PIPE command is
       interrupted by Ctrl/Y, deletes the PIPE command context.

    o  THEN, ELSE, ENDIF, SUBROUTINE, ENDSUBROUTINE, RETURN, and
       DCL labels - These commands cannot execute as PIPE command
       sequences because it is not possible to realize their
       functions in a PIPE command context.