Sort of triggered by what CountZero and chromatic said, I tried this:

use strict;
use warnings;
use Data::Dumper;
use threads;

{
    my @ReturnData = sub1();
    print Dumper(@ReturnData);
}
{
    my $ReturnData = sub1();
    print Dumper($ReturnData);
}
{
    my $thr = threads->new(\&sub1);
    my @ReturnData = $thr->join;
    print Dumper(@ReturnData);
}
{
    my $thr = threads->new(\&sub1);
    my $ReturnData = $thr->join;
    print Dumper($ReturnData);
}
{
    my ($thr) = threads->new(\&sub1);
    my @ReturnData = $thr->join;
    print Dumper(@ReturnData);
}
{
    my ($thr) = threads->new(\&sub1);
    my $ReturnData = $thr->join;
    print Dumper($ReturnData);
}

sub sub1
{
    return wantarray ? (1,2,3) : [4,5,6];
}
[download]

Results:

$VAR1 = 1;
$VAR2 = 2;
$VAR3 = 3;
$VAR1 = [
          4,
          5,
          6
        ];
$VAR1 = [
          4,
          5,
          6
        ];
$VAR1 = [
          4,
          5,
          6
        ];
$VAR1 = 1;
$VAR2 = 2;
$VAR3 = 3;
$VAR1 = 3;
[download]

The result makes 100% sense under the current design, as it matches what the document said, the context (whether wantarray) has been already determined at the thread creation time.