In summary, my questions are:

Are you running Windows in a VM? That's the only thing that comes to mind that might explain your results. I cannot reproduce them at all

I only have 4 cores, and the result from your script (slightly tweaked) show an almost perfect split of processing:

#! perl -slw
use strict;
use threads;
use Time::HiRes 'time';

our $T //= 4;

my @threads;
my $start = time;

foreach my $i (1 .. $T ) {
    $threads[$i] = threads->create(\&Work, $i);
}
foreach my $i ( 1 .. $T ) {
    $threads[$i]->join();
}

my $stop = time - $start;
printf "\nclock: %f sec user: %f\n", $stop, (times())[0];

exit;

#####

sub Work {
    my ($i) = @_;

    foreach ( 1 .. ( 20e5 / $T ) ) {
        my $acct_nrs = "abc\txyz\tdef\tabc\tghi\tghi";
        my @temp = split(m/\t/, $acct_nrs, -1);
        @temp = ( sort keys %{{ map { $_ => 1 } @temp }} );
        my $ans = join(', ', @temp);
    }
    printf " $i";
    return;
}

__END__
C:\test>for /l %i in (1,1,4) do @972137 -T=%i
 1
clock: 29.351637 sec user: 29.093000
 1 2
clock: 14.986346 sec user: 29.765000
 1 2 3
clock: 10.131188 sec user: 29.968000
 2 3 4 1
clock: 7.781729 sec user: 29.796000
[download]

Update: Ditto for 5.14:

C:\test>for /l %i in (1,1,4) do @\perl64-14\bin\perl.exe -slw 972137.p
+l -T=%i
 1
clock: 27.309776 sec user: 27.343000
 2 1
clock: 13.878018 sec user: 27.625000
 2 1 3
clock: 9.370494 sec user: 27.875000
 3 2 1 4
clock: 7.205594 sec user: 27.765000
[download]