In terms of the relative speeds, running the code through B::Concise is maybe helpful.

I can't interpret the full details, but the code to return a list of keys has fewer operations and only one nextstate (so less bookkeeping?). Both have the same number of ops that are optimised away (those preceded by "ex-").

Hopefully someone with better knowledge of the internals and B::Concise can shed some light.

C:\path>perl -v | find "This is"
This is perl 5, version 28, subversion 0 (v5.28.0) built for MSWin32-x
+64-multi-thread
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C:\path>type return_hash_keys.pl
use strict;
use warnings;
sub zort {
  my %hash = (a => 1);
  return keys %hash;
};
my @x = zort();
#my $aa = $x[0];

C:\path>perl -MO=Concise,-src return_hash_keys.pl
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
# 7: my @x = zort();
2     <;> nextstate(main 6 return_hash_keys.pl:7) v:*,&,{,x*,x&,x$,$ -
+>3
9     <2> aassign[t2] vKS/COM_AGG ->a
-        <1> ex-list lK ->7
3           <0> pushmark s ->4
6           <1> entersub lKS/STRICT ->7
-              <1> ex-list lK ->6
4                 <0> pushmark s ->5
-                 <1> ex-rv2cv sK/STRICT,1 ->-
5                    <#> gv[IV \&main::zort] s ->6
-        <1> ex-list lK ->9
7           <0> pushmark s ->8
8           <0> padav[@x:6,7] lRM*/LVINTRO ->9
return_hash_keys.pl syntax OK
[download]

C:\path>type return_hash_ref.pl
use strict;
use warnings;
sub zort {
    my %hash = (a => 1);
    return \%hash
};
my $x = zort();
my @x = keys %$x;
#my $aa = $x[0];

C:\path>perl -MO=Concise,-src return_hash_ref.pl
g  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
# 7: my $x = zort();
2     <;> nextstate(main 6 return_hash_ref.pl:7) v:*,&,{,x*,x&,x$,$ ->
+3
7     <2> sassign vKS/2 ->8
5        <1> entersub sKS/STRICT ->6
-           <1> ex-list sK ->5
3              <0> pushmark s ->4
-              <1> ex-rv2cv sK/STRICT,1 ->-
4                 <#> gv[IV \&main::zort] s ->5
6        <0> padsv[$x:6,8] sRM*/LVINTRO ->7
# 8: my @x = keys %$x;
8     <;> nextstate(main 7 return_hash_ref.pl:8) v:*,&,{,x*,x&,x$,$ ->
+9
f     <2> aassign[t6] vKS/COM_AGG ->g
-        <1> ex-list lK ->d
9           <0> pushmark s ->a
c           <1> keys[t5] lK/1 ->d
b              <1> rv2hv[t2] lKRM/STRICT ->c
a                 <0> padsv[$x:6,8] sM/DREFHV ->b
-        <1> ex-list lK ->f
d           <0> pushmark s ->e
e           <0> padav[@x:7,8] lRM*/LVINTRO ->f
return_hash_ref.pl syntax OK
[download]