use strict;
use warnings;
use Benchmark qw(timethese);

sub orig {
   for my $i ( 0 .. 9 ) {
      for my $j ( 0 .. 9 ) {
         for my $k ( 0 .. 9 ) {
            for my $l ( 0 .. 9 ) {
               my $filename = "$i$j$k$l";
            }
         }
      }
   }
}

sub yourmum {
   for ( 0 .. 9999 ) {
      my $filename = sprintf "%04d", $_;
   }
}

sub marshall {
   for ('0000' ... '9999') {
      my $filename = $_;
   }
}

orig();
yourmum();
marshall();

timethese 50000, {
   Orig     => sub { orig() },
   YourMum  => sub { yourmum() },
   Marshall => sub { marshall() },
};