The when ([wxID_YES]) {....} also works (why?)

I also wondered that. :-) Here is the answer I came up with:

1. From Experimental Details on given and when, it is clear that this is not one of the “10 exceptional cases” in which the expression is treated as a boolean. Therefore, smartmatching applies.

2. From Smartmatch Operator:

   The smartmatch implicitly dereferences any non-blessed hash or array r
   +eference, so the HASH and ARRAY entries apply in those cases.
   [download]

3. Also from Smartmatch Operator:

   Right operand is an ARRAY:
   
   ...
   
   Any       ARRAY      smartmatch each ARRAY element
                  like: grep { Any ~~ $_ } ARRAY
   [download]

4. and

   Num       nummy       numeric equality
                   like: Num == nummy
   [download]

   where nummy is defined as

   Either an actual number, or a string that looks like one.
   [download]

So,

creates an anonymous array reference, and populates the array with the result of calling wxID_YES() — namely, 5103 — then it dereferences the array reference (1) & (2), and performs a smartmatch on the elements (3), effectively:

grep { $selection ~~ $_ } (5103)
[download]

which (4) reduces to:

$selection == 5103
[download]

which actually succeeds in being DWIM.

All clear now?   ;-)

*   *   *

Also interesting is tobyink’s observation about constant, because

use constant FOO => 42;
[download]

is implemented as a subroutine named FOO. The significant point is that this subroutine is prototyped to take no arguments:

sub FOO() { return 42; }
[download]

which (together with some other requirements) allows Perl to inline it — that is, wherever FOO appears in the code (outside of quotes), it is replaced with the value 42, so smartmatching just works as expected. See Constant Functions.