use strict;
use warnings;

my @lines;

{
   local *FILE;

   open(FILE, "< filename.in")
   or die("Couln't open input file: $!\n");

   chomp(@lines = <FILE>);
}

my @sorted_lines = map  { $_->[0]}
                   sort { $a->[1] <=> $b->[1] }
                   map  { /^(\d+)/ && [ $_, 0+$1 ] }
             # OR: map  { no warnings; [ $_, 0+$1 ] }
                   @lines;

{
   local *FILE;

   open(FILE, "> filename.out")
   or die("Couln't open output file: $!\n");

   local $, = "\n";
   print FILE (@sorted_lines);
}