"No +1 does no work either"

Sure it does:

use warnings;
use strict;

#               0   1   2   3   4   5   6   7
#               '   '   '   '   '   '   '   '
my @words = qw(aaa bbb ccc ddd eee fff ggg hhh);
my @tst_num_temp = qw(0 4 6);

my $c2 = 0;
my @tst_num;

while ($c2<=$#tst_num_temp) {
  $tst_num[$c2]=$words[$tst_num_temp[$c2]+1];
  $c2++;
}

print "@tst_num\n";
[download]

...prints:

bbb fff hhh
[download]

"but I got the answer we can do it like $words[++$tst_num_temp[$c2]"

You can, and it will get you the exact same result for @tst_num as the +1 method. But in addition to that it will also modify @tst_num_temp, which you should be aware of in case the following code continues to use that variable.

In any case, consider using a for loop like 2teez suggested, it's much nicer compared to a while loop with manually incremented index.