Also if the id's are both small and numeric...

use strict;
use warnings;

my @myarray;
while(<>)
{
 chop;
 my($id, $data) = $_ =~ /(\d+)\s(.*)/;
 push (@{$myarray[$id]}, "$data");
}
 
for(my $i=0;$i<@myarray;$i++)
{
 next unless defined($myarray[$i]);
 foreach my $data ( @{$myarray[$i]} )
 {
  print "$i $data\n";
 }
}
[download]

This does the above and also the data is sorted. However, if the id numbers are large this will run you out of memory because perl will allocate the array out to the largest id. Also any id which is not present in the data will leave an undef in its position in the array. These need to be checked for and skipped or you will get warnings.

This method basically reads the entire file into memory in an organized format and then dumps it back out in id order. Note thought that if you have large id numbers and they are sparse, i.e. most are unused, then the memory will be larger than the hash method. If however the id numbers are dense and numeric this will be pretty memory efficient.

As also mentioned before by Abigail II, if you just want a sorted output file then the compiled utilities (sort) will probably an order of magnitude or so faster.