If the file is not so big that the keys will not fit into memory you can do this:

#!/usr/bin/perl
use strict;
use warnings;

use vars qw( @data $prev $IN @sorted $OUT);

$prev = 0;
open($IN, '<', 'data');
while (<$IN>) {
    chomp;
    my $next = tell($IN);
    my ($key) = /^([^\s]*)/;
    push(@data, [ $key, $prev, $next - $prev ]);
    $prev = $next;
}
close($IN);

@sorted = sort( { $a->[0] cmp $b->[0] } @data);

open($IN, '<', 'data');
open($OUT, '>', 'sorteddata');
for (@sorted) {
    seek($IN, $_->[1], 0);
    sysread($IN, my $line, $_->[2]);
    print $OUT $line;
}
close($IN);
close($OUT);
[download]

On the cooked data I tested, I got the following timings:

Gnu Sort:
# time sort --temporary-directory=/opt data > sort1

real    0m24.698s
user    0m22.539s
sys     0m1.950s

Perl Sort:
# time perl sort.pl 

real    0m55.900s
user    0m39.897s
sys     0m6.430s

The data file I used had a wc of:

#wc data
  4915200  34406400 383385600 data

I am surprised that this Perl script is only half the speed of Gnu sort on this data. I think that on a bigger data set, with long lines, it might even be able to sort faster that Gnu Sort.

UPDATE: Most of the time seems to be being spent in the output loop. All of the seeks seems to really slow things down.

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