As your data looks to be time sequential something easy like this would probably do you fine. I have given a simple way to do it with two hashes or a more perlish way with a hash of arrays. If your data lines are not ordered by time then a check to compare two time stamps will be needed for each line to decide if to update first or last time.

#!/usr/opt/perl5/bin/perl
use strict;
use warnings;
use Data::Dumper;
 
my %earliest;
my %latest;
my %hash_of_arrays;
 
my $line;
while ($line = <DATA>) {
    chomp $line;                              # remove return
    my ($key, $date) = split /\s+/, $line, 2; # split on space into ma
+x 2 parts
    next unless $date;                        # ignore blank or malfor
+med lines
    unless (defined $earliest{$key}) {
        $earliest{$key} = $date;
    }
    $latest{$key} = $date;
 
    # or with a hash of arrays
 
    $hash_of_arrays{$key} or $hash_of_arrays{$key}[0]=$date;
    $hash_of_arrays{$key}[1]=$date;
}
 
print Data::Dumper->Dump([\%earliest], ['%earliest']) ;
print Data::Dumper->Dump([\%latest], ['%latest']);
print Data::Dumper->Dump([\%hash_of_arrays], ['%hash_of_arrays']);
print "first time for KEY1 was: $hash_of_arrays{KEY1}[0]\n";
print "last  time for KEY1 was: $hash_of_arrays{KEY1}[1]\n";

 
__DATA__
KEY1 07/25/05 09:04:36
KEY2 07/25/05 09:04:36
KEY2 07/25/05 09:04:37
KEY2 07/25/05 09:05:49
KEY2 07/25/05 09:05:50
KEY3 07/25/05 09:05:50
KEY1 07/25/05 09:06:36

# output
=>./first 
$%earliest = {
               'KEY2' => '07/25/05 09:04:36',
               'KEY1' => '07/25/05 09:04:36',
               'KEY3' => '07/25/05 09:05:50'
             };
$%latest = {
             'KEY2' => '07/25/05 09:05:50',
             'KEY1' => '07/25/05 09:06:36',
             'KEY3' => '07/25/05 09:05:50'
           };
$%hash_of_arrays = {
                     'KEY2' => [
                                 '07/25/05 09:04:36',
                                 '07/25/05 09:05:50'
                               ],
                     'KEY1' => [
                                 '07/25/05 09:04:36',
                                 '07/25/05 09:06:36'
                               ],
                     'KEY3' => [
                                 '07/25/05 09:05:50',
                                 '07/25/05 09:05:50'
                               ]
                   };
first time for KEY1 was: 07/25/05 09:04:36
last  time for KEY1 was: 07/25/05 09:06:36
[download]

Cheers,
R.