well yeah.. you're doing some weird kind of declaring your variables. I tried it with an array.

#!/usr/bin/perl -w
use threads;
my @th;
for($i=0;$i<10;$i++){
    push @th, threads->new(\&sub2,$i) ;
}

for($i=0;$i<10;$i++) {
    $th[$i]->join;
    #$result = $th[$i]->eval;
    $th[$i]->detach;
}

sub sub2 :locked {
    my @para = @_;
    my $t = $_[0];
    my $temp="test";
    my $user=$temp.$t;
    if (!defined($pid = fork)) {
        die "Cannot fork\n";
    } elsif ($pid == 0 ) {
        exec("echo $user");
    } else {
        waitpid($pid,0);
    }
}
[download]

the output is, yes.. indeed:

test0
test1
test2
test3
test4
test5
test7
test8
test9
test6
[download]

i couldn't find anything in perldoc threads about the ->eval you have in your code.

but maybe you already solved your problem..