Those bits of source show how strict vars works, but what I'm more interested (and couldn't figure out) is which part of the code determines whether or not a glob assignment will result in setting the IMPORT flag on the GV.

Well, http://perl5.git.perl.org/perl.git/blob?f=gv.c#l1578 is it I think, HINT_STRICT_VARS means strict is on, and that is where the name/flags are checked  (*name == 'a' || *name == 'b') should be familiar (its $a and $b)

Its not commented/decorated, so my next move would be to fireup gitk or some such gitness to see how that block has changed (if at all) over the years ... and refresh my http://search.cpan.org/dist/illguts/index.html knowledge ... cause whatever I knew once I forgot :)

Then

Peeking at the array is meh

Peeking at the glob is more promising

$ perl -e "use Devel::Peek; use strict; BEGIN{*main::foo=\@main::foo; 
+Dump(*main::foo)};  print @foo; "
SV = PVGV(0xa6ad1c) at 0x99b984
  REFCNT = 4
  FLAGS = (MULTI,IN_PAD)
  NAME = "foo"
  NAMELEN = 3
  GvSTASH = 0x3f991c    "main"
  GP = 0xac5fac
    SV = 0x0
    REFCNT = 1
    IO = 0x0
    FORM = 0x0
    AV = 0x99ba04
    HV = 0x0
    CV = 0x0
    CVGEN = 0x0
    LINE = 1
    FILE = "-e"
    FLAGS = 0xa
    EGV = 0x99b984      "foo"
Variable "@foo" is not imported at -e line 1.
Global symbol "@foo" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.

$ perl -e "use Devel::Peek; use strict; BEGIN{package Pkg; *main::foo=
+\@main::foo;};package main; Dump*foo; "
SV = PVGV(0xa6ad3c) at 0x99b994
  REFCNT = 2
  FLAGS = (MULTI,IN_PAD,IMPORT( AV ))
  NAME = "foo"
  NAMELEN = 3
  GvSTASH = 0x3f9934    "main"
  GP = 0xac5fec
    SV = 0x0
    REFCNT = 1
    IO = 0x0
    FORM = 0x0
    AV = 0xa8d714
    HV = 0x0
    CV = 0x0
    CVGEN = 0x0
    LINE = 1
    FILE = "-e"
    FLAGS = 0x2a
    EGV = 0x99b994      "foo"
[download]

Something something ... looking at prev link ... something http://perl5.git.perl.org/perl.git?a=search&h=HEAD&st=grep&s=GvIMPORTED

http://search.cpan.org/dist/illguts/index.html#GvFLAGS

http://perl5.git.perl.org/perl.git?a=search&h=HEAD&st=grep&s=IMPORTED\S%2Bon&sr=1
http://perl5.git.perl.org/perl.git/blob?f=gv.h#l178
?imported subs http://perl5.git.perl.org/perl.git/blob?f=gv.c#l399
?imported scalar http://perl5.git.perl.org/perl.git/blob?f=pp_hot.c#l161
?imported glob scalar assignment http://perl5.git.perl.org/perl.git/blob?f=sv.c#l3758
?imported scalar ref http://perl5.git.perl.org/perl.git/blob?f=sv.c#l4234
!SCREAM http://perl5.git.perl.org/perl.git/blob?f=sv.h#l362

360 #define SVprv_PCS_IMPORTED  SVp_SCREAM  /* RV is a proxy for a con
+stant
361                                        subroutine in another packa
+ge. Set the
362                                        GvIMPORTED_CV_on() if it ne
+eds to be
363                                        expanded to a real GV */
[download]

So, next move, somehow gitk/gitness , track IMPORTED\S+on changes ... maybe just grep each major release ..

When lacking understanding, I grep more :D