Let us call the hash

    my %ip_address;
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Once you have read to the end of the data, your hash will contain the first occurances of each IPaddress+action. This pseudo-code (note: it only looks vaguely like Perl, not tested) shows you how to extract and sort to get the final report:

foreach my $data_line ( sort values %ip_action ) {
    print($data_line);
}
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my %see;
open my $file, "<textfile.txt";
while ( my $line = <$file> ) {
    my ($date, $rest) = split(" ",$line,2);
    next if $see{$rest}++;
    print "$date $rest";
}
close $file;
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The easiest way is probably to split your line on space, tab or comma depending on what your file uses, take all the fields beyond the date and time and use them as a hash key join will glue them back together for you. If the key already exists you have seen this line before and can discard it, it not write the line out, create the key and move onto the next line.

There are a few optimistaions depending on your need for speed, code simplicity or fun and games. The first is to only split the line into three, Date, Time, Rest and the you do not have to join back up. another is to test if the hash key exists at the same time you create it:

if ($hash{$key}++) {
    # key already seen
}else{
    # key not yet seen
}
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Have a go and post your code if you have any problems

Cheers,
R.

Adjust the number to the length of your timestamps.

uniq -us 20 infile outfile
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If you're on windows and do not have access to uniq, get it here amongst other places.

However the IPaddress and Action part of each line may contain duplicates, its these duplicates I want to remove but still keep the output in time order.

If you keep the first and delete later repeats, you just keep the IP/Action data as hash keys, and assuming the data are being read in chronological order, only output lines whose IP/Action are not yet in the hash.

In order to delete earlier occurrences and keep only the latest one, you have to store Date/Time as the value for each IP/Action key, and after you've read the whole input stream, sort the hash by its values in order to print each "hash_value hash_key" in chronological order.