No shell is involved. If you strace perl (strace -f -ff on linux), you'll see that no shell is spawned.

AFAIK perl now uses File::Glob internally¹, and in fact foo.pl as

$a=q|a\ \\\{bc.d/*|;print "pattern <", $a, "> globs to <", (glob $a), 
+">\n";
print "$_\n" for sort keys %INC;
[download]

yields

pattern <a\ \\{bc.d/*> globs to <a {bc.d/GOT ITabcde>
Carp.pm
Exporter.pm
File/Glob.pm
Text/ParseWords.pm
XSLoader.pm
strict.pm
vars.pm
warnings.pm
warnings/register.pm
[download]

while running

$a=q|a\{bc.d/*|;print "pattern <", $a, "> globs to <", (glob $a), ">\n
+";
print "$_\n" for sort keys %INC;
[download]

results in

pattern <a\{bc.d/*> globs to <a{bc.d/GOT ITabcde>
File/Glob.pm
XSLoader.pm
strict.pm
[download]

I suspect a subtlety (bug?) in Text::ParseWords or its usage. The behaviour you describe (which I confirm for my systems) looks like a two-pass parsing whenever a whitespace is present - in that case the \{ seems to get "optimized away" (running with -Dcr):

Guessing start of match, REx "\\(.)" against "\{bc.d/*"...
Found anchored substr "\" at offset 0...
Guessed: match at offset 0
Matching REx "\\(.)" against "\{bc.d/*"
  Setting an EVAL scope, savestack=56
   0 <> <\{bc.d/*>        |  1:  EXACT <\\>
   1 <\> <{bc.d/*>        |  3:  OPEN1
   1 <\> <{bc.d/*>        |  5:  SANY
   2 <\{> <bc.d/*>        |  6:  CLOSE1
   2 <\{> <bc.d/*>        |  8:  END
Match successful!
Guessing start of match, REx "\\(.)" against "bc.d/*"...
Did not find anchored substr "\"...
Match rejected by optimizer
Not present...
Match failed
[download]

No time to track that down rigth now... smells like eval involved.

¹) From doio.c:

=head1 IO Functions

=for apidoc start_glob

Function called by C<do_readline> to spawn a glob (or do the glob insi
+de
perl on VMS). This code used to be inline, but now perl uses C<File::G
+lob>
this glob starter is only used by miniperl during the build process.
Moving it away shrinks pp_hot.c; shrinking pp_hot.c helps speed perl u
+p.

=cut
*/

PerlIO *
Perl_start_glob (pTHX_ SV *tmpglob, IO *io)
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--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}