1. walk through threads
2. each key in threads points to an array ref
3. store each arrayref into an array

modifying your code without munging it too much, I change to storing the array ref in $tlist[$tnum]; And then when we access from $tlist, we dereference into the array.

The syntax you had @tlist[$i] was actually doing an array slice.

print "\n\nThe Hash Table\n\n";
foreach $thread (keys %threads) {
    print "$thread: @{$threads{$thread}}\n";
}

#this prints out lines like:
#keyA: 2 6 8
#keyB: 3 5 10 14 18
#etc.

# Convert hash table into arrays for Threads, with common index $tnum

$tnum=0;
foreach $thread (keys %threads) {
    $tsub[$tnum] = $thread;    
    $tlist[$tnum] = $threads{$thread}; #store the array reference. 
    $tsize[$tnum] = @{ $tlist[$tnum] };    
    $tnum++;
}

print "\n\nThe Thread Arrays\n\n";
for ($i=0;$i<$tnum ;$i++) {
    print "\n$tsub[$i], size=$tsize[$i], @{$tlist[$i]";
}
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