Just as a general point, rather than "sort then filter", it's generally a better idea to "filter then sort". This is because sorting is a comparatively slow operation; filtering first reduces the size of the list that needs to be sorted, making the sorting faster.

You won't notice any difference on a list with 25 items, but if you've got thousands, it can make a significant difference.

Yes, I fully agree with tobyink, filtering first and then only sorting can generally improve performance dramatically because the program will have much less work to do (at least if you have many records). In this specific case, filtering on today's date offers another major advantage: since all the dates will be the same, you really have to sort on time, and this is much simpler and easier thah sorting dates in your given format.

If you had to sort on dates and then times, you would need to split the records in order to sort first the years, then the months, then the days, and then the time (or possibly apply some other type of transformation to your data). Here, since all the dates will have been filtered and will therefore be the same, a simple sort on the full record will produce the desired result.

So you basically need something as simple as this:

my @sorted_records = sort grep {/^08-14-2013/} @unsorted_records;

Of course, in real life, you probably don't want to hardcode the date (08/14/2013) in your filtering. If you want to find today's date in your specific date format, you can do something like this:

my ($day, $month, $year) = (localtime time)[3..5];
$year += 1900;
$day = sprintf "%02d", $day:
$month = sprintf "%02d", $month + 1;
my $date = "$month-day-$year";
[download]

Or you could use one of the numerous date modules available on the CPAN. Or the strftime POSIX function illustrated above by boftx.

Based on what was just said, my example can be simplified even further:

# this is what should be used instead of the hard-coded value:
# my $today = strftime("%m-%d-%Y",localtime());
my $today = '08-14-2013';

my @todays;
for ( @orig_data ) {
    next unless m/^($today )/;

    push(@todays,$_);
}

# NOTE! this will reverse the order of Yes and No if they have the
# same time.
@todays = sort(@todays);

for ( @todays ) {
    print "$_\n";
}
[download]

I agree that filtering then sorting will result in (sometimes large) performance gains. In this particular case, where the unsorted array is actually records coming from a file, you only want to store the records of interest in memory instead of slurping in the entire file. RAM might be cheap, but it is a finite resource and SAs can be a nasty breed of cat. (Remember the BOFH?)

That is known simply as a sort

perlintro, String comparison, Simple matching

http://learn.perl.org/faq/perlfaq4.html#How-do-I-sort-an-array-by-anything-, http://perldoc.perl.org/perlfaq4.html#How-do-I-sort-an-array-by-%28anything%29%3f

gmtime/localtime/Time::Piece (strptime/strftime)/POSIX::strftime

#!/usr/bin/perl --
use strict;
use warnings;
use Time::Piece;

Main( @ARGV );
exit( 0 );

sub Main {
    my( $now ) = @_;
##    $now ||= Time::Piece::localtime->strftime('%m-%d-%Y');
    $now  ||= "08-13-2013";
    my $raw =
'Date        Time      Tag
08-13-2013 21:22:17  Yes
08-13-2013 22:22:17  No
08-14-2013 11:22:17  Yes
08-13-2012 21:22:17  Yes
08-13-2011 22:22:17  No
08-14-2012 01:22:17  Yes
';;;;
    my( $header, $todaytes ) = rubberBiscuit( $now , \$raw );
    print $header;
    print "$_\n" for @$todaytes ;
}
sub rubberBiscuit {
    my( $now, $file ) = @_;
    use autodie;
    open my($in), '<', $file ; ## or die by autodie
    my @today;
    my $header = readline $in;
    while( readline $in ){
        my($date, $time, $tag ) = split ' ';
        if( $date eq $now ){
            push @today,
            join ' ',
                Time::Piece->strptime(
                    "$date $time", '%m-%d-%Y %H:%M:%S',
                )->strftime('%Y-%m-%d %H:%M:%S'),
                $tag,
            ;;;;;
        }
    }
    @today = sort @today;
    return $header, \@today;
}

__END__
Date        Time      Tag
2013-08-13 21:22:17 Yes
2013-08-13 22:22:17 No
[download]

I think this can be simplified somewhat. The following just demonstrates the basic selection/sort logic without regard to how you want to handle the file itself.

#!/usr/bin/perl

use strict;

use POSIX qw(strftime);

my @orig_data = (
'Date        Time      Tag',
'08-13-2013 19:22:16  Yes',
'08-13-2013 18:22:17  No',
'08-13-2013 21:22:17  Yes',
'08-13-2013 20:22:16  Yes',
'08-13-2013 23:22:18  Yes',
'08-13-2013 22:22:17  No',
'08-14-2013 01:22:17  Yes',
'08-14-2013 00:22:18  Yes',
'08-14-2013 03:22:19  No',
'08-14-2013 02:22:18  Yes',
'08-14-2013 05:22:28  No',
'08-14-2013 04:22:29  Yes',
'08-14-2013 07:22:19  Yes',
'08-14-2013 06:22:18  Yes',
'08-14-2013 09:22:19  No',
'08-14-2013 08:22:19  Yes',
'08-14-2013 11:22:19  Yes',
'08-14-2013 10:22:20  No',
'08-14-2013 13:22:20  Yes',
'08-14-2013 12:22:20  No',
'08-14-2013 15:22:21  Yes',
'08-14-2013 14:22:20  Yes',
'08-14-2013 17:22:21  Yes',
'08-14-2013 16:22:22  No',
);

# this should be used instead of the hard-coded value below:
# my $today = strftime("%m-%d-%Y",localtime());
my $today = '08-14-2013';

my @todays;
for ( @orig_data ) {
    my ($date,$other) = split(/\s/,$_,2);
    next unless $date eq $today;

    push(@todays, "$other");
}

# NOTE! this will reverse the order of Yes and No if they have the sam
+e time.
@todays = sort(@todays);

for ( @todays ) {
    print "$today $_\n";
}

__END__
[download]

• if you need to run this just once, run time does not matter as much as programming time
• if specifications/requirements change relatively often (or are not yet stable), it is more important to have code that is easy to understand than code that is efficient

Of course, what is really being done is this:

$ grep `date +%m-%d-%Y` datafile.txt | sort >data_`date +%m-%d-%Y`.txt
[download]

But who wants to use a scripting language? :)