use Benchmark;
timethese (1000000, {
     'substr' => sub { my $string = '123456789B31S';
                       $string = substr($string,4,(length($string)-6)); },
     'regexp' => sub { my $string = '123456789B31S';
                       $string =~ s/^.{4}(.+).{2}$/$1/; }});