Your problem is that the return context (scalar/array/void) of the thread subroutine, is defined by the context in which that thread is created.

In your example, that means here:

my $thr = threads->new(\&sub_returning_hash, $argument);
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Which is a scalar context. Hence, when the return is invoked here:

return %output_record;
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It is the equivalent of

return scalar %output_record;
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Which returns:

%h = qw[ a 11 b 12 c 13 ];;

print scalar %h;;
3/8
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And when that string is returned from the join and assigned to the hash in the calling code:

%r = scalar %h;;
Odd number of elements in hash assignment
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A complicated explanation, but thankfully, the solution is simple. Change the thread creation line to:

my( $thr ) = threads->new(\&sub_returning_hash, $argument);
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Which puts the creation of the thread into an array (list) context.

Now the code will run warnings free and produce the desired result:

[11:51:36.25]C:\test>junk
a = 11
b = 12
c = 13
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Alternatively, you can mark the return context of the thread sub explicitly at creation time:

my $thr = threads->new( {'context' => 'list'}, \&sub_returning_hash, $
+argument );
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which will achieve the same result. See the section labelled "THREAD CONTEXT" in the threads pod for details.