You don't really need to worry about it, because perl's concatenation operator is optimized for that.

use Time::HiRes qw( time );
for my $exp (3 .. 8) {
   my $count = 10**$exp;
   my $start = time();
   my $x = '';
   for (1 .. $count) {
      $x .= 'x';
   }
   printf "%d  %.5f\n", $count, time() - $start;
}
[download]

1000  0.00012
10000  0.00113
100000  0.01161
1000000  0.11535
10000000  1.09300
100000000  11.02793
[download]