perl -le "open(my $x, "">&STDOUT"");"

Not quite - but close enough ;-)
Thank you.
Either of the following work and, AFAICT, are equivalent - though I'm not exactly sure why that equivalence should hold:

perl -le "open(my $x, \"">&STDOUT"\");"
perl -le "open(my $x, "\">&STDOUT\"");"
[download]

So I gather it's just the way the shell functions (as opposed to an issue with perl), that I can do:

C:\_32>perl -Mstrict -MDevel::Peek -le "my $arg = \"STDOUT\"; Dump $ar
+g;"

SV = PV(0x36cd1c) at 0x44b124
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x451dc4 "STDOUT"\0
  CUR = 6
  LEN = 10
  COW_REFCNT = 1
[download]

but it blows up as soon as I introduce shell metacharacters into the string:

C:\_32>perl -Mstrict -MDevel::Peek -le "my $arg = \">&STDOUT\"; Dump $
+arg;"
>& was unexpected at this time.
[download]

And I guess the additional quotes provide the disambiguation required to allow the one-liner to DWIM.

Cheers,
Rob