Untested:

perl -F\t -anle"($t)=$F[0]=~/^(.{5})/;$t=~s/(.)$/$1<5?0:5/e; $t{$t}=$F
+[1] if $F[1]>$t{$t} }{ print qq[$_\t$t{$_}] for sort keys %t" file.cs
+v
[download]

Construct a hash of arrays keyed by value in the file, containing arrays of lines for each value in case there is more than one time period with the same value. Use List::Util->max to find the largest key in the hash and print the line(s) associated with it.

knoppix@Microknoppix:~$ perl -Mstrict -Mwarnings -MList::Util=max -E '
> open my $fh, q{<}, \ <<EOD or die $!;
> 10:38:34   759
> 10:43:34   735
> 10:48:34   817
> 10:53:34   648
> 10:58:34   817
> 11:03.34   774
> EOD
> 
> my %byValue;
> while ( <$fh> )
> {
>     my $value = ( split )[ 1 ];
>     push @{ $byValue{ $value } }, $_;
> }
> 
> print for @{ $byValue{ max keys %byValue } };'
10:48:34   817
10:58:34   817
knoppix@Microknoppix:~$
[download]

I hope this is helpful.

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

open FIN,  '<', 'something.csv'     or die "Cannot open 'something.csv
+' because: $!";
open FOUT, '>', 'something_new.csv' or die "Cannot open 'something_new
+.csv' because: $!";

my %data;
while ( <FIN> ) {
    next unless /^(\d+):(\d+):\d+\s+(\d+)$/;
    my $key = sprintf '%02d%02d', $1, int( $2 / 5 ) * 5;
    if ( exists $data{ $key } ) {
        $data{ $key } = [ $., $_, $3 ] if $data{ $key }[ 2 ] < $3;
        }
    else {
        $data{ $key } = [ $., $_, $3 ];
        }
    }

print FOUT map $_->[ 1 ], sort { $a->[ 0 ] <=> $b->[ 0 ] } values %dat
+a;
[download]

Below is the code I am working with.

That is data, not code. Post your code.

What is the best way of doing this?

:) Hire a programmer, or just do it already :)

Use Text::CSV or Text::xSV

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