Nice! If I'm not mistaken, you can shorten it still: 

use strict;
use warnings;
my $previous = '';

print
grep { $_ }
map  {
    if ($previous ne $_->[1]) {
        $previous = $_->[1];
        $_->[0];
    }
}

sort {
    $a->[1] cmp $b->[1] or $b->[2] <=> $a->[2]
}

map {
    my ($date, $magnitude) = (split/,/)[3,4];
    [ $_,
      join("", (split(/[-:. ]/,$date))[2,1,0,3,4,5]),
      $magnitude
    ]
}

<DATA>;

__DATA__
2550,531,66,10-12-2007 07:03:08.069,2
2549,529,62,10-12-2007 07:03:08.151,1
2550,531,66,10-12-2007 07:03:09.069,1
2549,529,62,10-12-2007 07:03:09.151,10
2549,529,62,10-12-2007 07:03:09.151,2
2549,529,62,10-12-2007 07:03:09.151,7
2549,529,62,10-12-2007 07:03:09.151,2
2549,529,62,10-12-2007 07:03:09.151,8
2549,529,62,10-12-2007 07:03:09.151,2
2549,529,62,10-02-2007 07:03:10.151,2
2549,529,62,10-12-2007 07:13:09.151,2
2549,529,62,10-12-2007 07:03:09.151,2
2549,529,62,10-12-2007 17:03:09.151,1

[download]

Gives me the same output as yours.

Update:Shortened a bit more and added strictures.

Update2: CountZero, thanks for pointing out the problem - it was subtle. I added a simple grep to fix it, which pretty much brings it back to your solution. Ah well, good stuff.

In reply to Re^2: Compare fields in a file by hbm
in thread Compare fields in a file by honyok

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