Interesting. It appears to be the result of compile-time optimization. (Update: Or maybe it's to generate the "useless use of a constant in void context" warning.)

sub wa {
    my $place = shift @_;
    my $context = wantarray ? 'list'
                : defined wantarray ? 'scalar' : 'void';
    warn "'$context' context at '$place'\n";
}

my $x = (wa('a1'),wa('a2'),wa('a3'));
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is compiled to

>perl -MO=Concise script.pl
j  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 4 script.pl:8) v ->3
i     <2> sassign vKS/2 ->j
g        <@> list sKP ->h
3           <0> pushmark v ->4
7           <1> entersub[t3] vKS/TARG,1 ->8
                             ^
                             |
                         void context

-              <1> ex-list K ->7
4                 <0> pushmark s ->5
5                 <$> const[PV "a1"] sM ->6
-                 <1> ex-rv2cv sK/1 ->-
6                    <#> gv[*wa] s ->7
b           <1> entersub[t5] vKS/TARG,1 ->c
                             ^
                             |
                         void context

-              <1> ex-list K ->b
8                 <0> pushmark s ->9
9                 <$> const[PV "a2"] sM ->a
-                 <1> ex-rv2cv sK/1 ->-
a                    <#> gv[*wa] s ->b
f           <1> entersub[t7] sKS/TARG,1 ->g
                             ^
                             |
                       scalar context

-              <1> ex-list sK ->f
c                 <0> pushmark s ->d
d                 <$> const[PV "a3"] sM ->e
-                 <1> ex-rv2cv sK/1 ->-
e                    <#> gv[*wa] s ->f
h        <0> padsv[$x:4,5] sRM*/LVINTRO ->i
script.pl syntax OK
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So,
if the context of the "," operator is known to be scalar at compile-time, the LHS is evaluated in void context.
if the context of the "," operator is known to be list at compile-time, the LHS is evaluated in list context.
if the context of the "," operator is not known at compile-time, the LHS is evaluated in the same context as the operator.