undef(@a) would give you want you want. undef isn't recursive. Undefining the glob doesn't undefine the values referenced by the glob.

Before glob assignment
----------------------

*main::a                                                 *main::b
   |                                                        |
   |    +-------+                              +-------+    |
   +--> | Glob  |                              | Glob  | <--+
        +-------+     +-------+  +-------+     +-------+
        | GP    ----> | GP    |  | GP    | <---- GP    |
        +-------+     +-------+  +-------+     +-------+
                    +-- ARRAY |  | ARRAY --+
        +-------+   | +-------+  +-------+ |
        | Array | <-+                      +-> NULL
        +-------+
        | 1,2,3 |
        +-------+


After glob assignment
---------------------

*main::a                                                 *main::b
   |                                                        |
   |    +-------+                              +-------+    |
   +--> | Glob  |                              | Glob  | <--+
        +-------+          +--------+          +-------+
        | GP    ---------> | GP     | <--------- GP    |
        +-------+          +--------+          +-------+
                           | ARRAY  --+
                           +--------+ |   +-------+
                                      +-> | Array |
                                          +-------+
                                          | 1,2,3 |
                                          +-------+
   
 
After "undef *a"
(Gets a fresh GP,
undoing the glob
assignment)
-----------------

*main::a                                                 *main::b
   |                                                        |
   |    +-------+                              +-------+    |
   +--> | Glob  |                              | Glob  | <--+
        +-------+     +-------+  +-------+     +-------+
        | GP    ----> | GP    |  | GP    | <---- GP    |
        +-------+     +-------+  +-------+     +-------+
                    +-- ARRAY |  | ARRAY --+
                    | +-------+  +-------+ |   +-------+
             NULL <-+                      +-> | Array |
                                               +-------+
                                               | 1,2,3 |
                                               +-------+


If you had done "undef @a"
(Clears @a)
--------------------------

*main::a                                                 *main::b
   |                                                        |
   |    +-------+                              +-------+    |
   +--> | Glob  |                              | Glob  | <--+
        +-------+          +--------+          +-------+
        | GP    ---------> | GP     | <--------- GP    |
        +-------+          +--------+          +-------+
                           | ARRAY  --+
                           +--------+ |   +-------+
                                      +-> | Array |
                                          +-------+
                                          | ()    |
                                          +-------+
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Note that "@a=();" is usually better than "undef @a;" since it doesn't discard @a's buffer.