or if you rather just retain the first and last time for each key for which there is more than one time:

use strict;
use warnings;
use Data::Dumper;

my %HoA;

while (<DATA>)
  {
  chomp;
  next if ! length $_;
  my ($key, $time) = split ",";
  if (! $HoA{$key})
    {push @{$HoA{$key}}, $time;}
  elsif (@{$HoA{$key}} == 1)
    {push @{$HoA{$key}}, $time;}
  else
    {${@{$HoA{$key}}}[1] = $time;}
  }
  
print Dumper (%HoA);
[download]

__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

$VAR1 = 'KEY2';
$VAR2 = [
          ' 07/25/05 09:04:36',
          ' 07/25/05 09:05:50'
        ];
$VAR3 = 'KEY1';
$VAR4 = [
          ' 07/25/05 09:04:36'
        ];
$VAR5 = 'KEY3';
$VAR6 = [
          ' 07/25/05 09:05:50'
        ];
[download]