I can't explain why some built-ins don't autovivify the hash, even though it needs to be flattened.

It's not about hash flattening, but the fact that arguments are passed into subroutines as references. So saying f( %{ $h } ); for an undefined $h looks up keys and values, which causes the container to spring into existence as a hash reference and be assigned to $h. That is necessary since the arguments to subs are potential candidates for assignment (lvalues)

perl -MO=Concise -le 'my $h;sub f {}  f( %{ $h } );'
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
3     <0> padsv[$h:1,4] vM/LVINTRO ->4
4     <;> nextstate(main 4 -e:1) v ->5
9     <1> entersub[t4] vKS/TARG,1 ->a
-        <1> ex-list K ->9
5           <0> pushmark s ->6
7           <1> rv2hv[t3] lKM/1 ->8
-              <@> scope sK ->7
-                 <0> ex-nextstate v ->6
6                 <0> padsv[$h:1,4] sM/DREFHV ->7
-           <1> ex-rv2cv sK/1 ->-
8              <#> gv[*f] s ->9
[download]

(note the line padsv[$h:1,4] sM/DREFHV with the M flag), which is not the case with print.

perl -MO=Concise -le 'my $h;print( %{ $h } )'
9  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
3     <0> padsv[$h:1,3] vM/LVINTRO ->4
4     <;> nextstate(main 3 -e:1) v ->5
8     <@> print vK ->9
5        <0> pushmark s ->6
7        <1> rv2hv[t2] lK/1 ->8
-           <@> scope sK ->7
-              <0> ex-nextstate v ->6
6              <0> padsv[$h:1,3] s ->7
[download]

Since print doesn't comprise modifcation of it's arguments, the hashref isn't created.

That's the most concise explanation of this oddity I can conceive up to now...

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