He was suggesting you don't have to do the reverse 7 times. I don't think that's true.

He also suggested you test the size of the variable and don't perform the substr() commands that will just return undef.

    my $start_run = time();
    use strict;
    use Benchmark;
    use warnings;
    my @array = (1..1000000000);
    foreach (@array) {
       for(my $i=1; $i<length($_);$i++) {
          if ($_==reverse(substr($_, 0, $i) * substr($_, $i)))
            { print $_."\n" ;}
       }
    }
    my $end_run = time();
    my $run_time = $end_run - $start_run;
    print "Time taken: ".$run_time." sec\n";
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