perlthrtut explicitly says that you can return a list, but the example code doesn't work as advertised:

use threads; 

$thr = threads->new(\&sub1);

@ReturnData = $thr->join; 
print "Thread returned @ReturnData"; 

sub sub1 { return "Fifty-six", "foo", 2; }
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prints "Thread returned 2". It looks like if the return statement returns a list you'll get the last element, and if you wrap it up inside an array (e.g., return @array = ("Fifty-six", "foo", 2)) it'll put it in scalar context no matter what the lvalue of the join(). Weird.

Another option is to return an array reference and assign it to a scalar left of the join, or to pass a reference to a shared array into the threaded sub and have it populated.

use threads;
use threads::shared;

share(@array);

$thr = threads->new("sub1");
@ReturnData = $thr->join; 
print "Thread returned @ReturnData\n"; 
print "Thread populated @array\n"; 

sub sub1 { @array = ("Fifty-six", "foo", 2) }
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prints "Thread returned 3" and "Thread populated Fifty-six foo 2".