Following BrowserUk's advice:

#!/usr/bin/perl

use threads;
use Data::Dumper::Concise;

sub sub_returning_hash {
    use strict; 
    no strict 'refs';
    use warnings;
    no warnings 'uninitialized';
    
    my $argument = shift();
    my $a = $argument + 1; 
    my $b = $argument + 2; 
    my $c = $argument + 3; 
    my(%output_record) = (
        a => "$a",
        b => "$b",
        c => "$c",
    );
    return %output_record;
} 

use strict;
use warnings;

my $argument = 10;
my($thr) = threads->create(\&sub_returning_hash, $argument);

my (%hash) = $thr->join();
print Dumper( "a = $hash{'a'}" );
print Dumper( "b = $hash{'b'}" );
print Dumper( "c = $hash{'c'}" );
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