What I'd do is simply use DB_File like so (untested, but hey, I wrote it ;))

#!/usr/bin/perl -w

use strict;
use Fcntl;
use DB_File;
use Date::Calc qw|Date_to_Time Decode_Month|;

my $filename = __FILE__.".${$}.db";

# $DB_BTREE is exported by DB_File
# it is a sorted balanced binary tree
# we enable duplicate keys
$DB_BTREE->{'flags'} = R_DUP ;

# since default sorting is lexical, and we want numeric
$DB_BTREE->{'compare'} = sub {
    my( $keyA, $keyB ) = @_;
    return $keyA <=> $keyB;
};


my $X = tie my(%H),
    "DB_File",
    $filename,
    O_RDWR|O_CREAT|O_TRUNC,
    0666,
    $DB_BTREE 
  or die "Cannot open $filename: $!\n";

foreach $file (@ARGV) {

    die("This is not a file: $!") unless (-f $file);

    open(FILE, $file) or die "WTF? couldn't open $file cause $!";
    while(<FILE>){
        chomp;
        my $date = get_timestamp($_);
        $H{$date} = $_;
    }
    close(FILE);
}

    # iterate through the btree using seq
    # and print each key/value pair.
    my $key = 0;
    my $value = 0 ;
    my $status = "";
    for( $status = $x->seq($key, $value, R_FIRST) ;
         $status == 0 ;
         $status = $x->seq($key, $value, R_NEXT)
    ) {
        print "$value\n";
    }
    undef $X ;
    untie %H ;

# delete our temporary database
unlink $filename or die "couldn't delete $filename cause $!";
exit; ## subland

sub get_timestamp {
    my $line = shift;

    $line =~ /\[(.*) -\d+\]/;

    my $tempdate = $1;

    my ($day,$mon,$year,$hour,$min,$sec) = 
                 $tempdate =~ /(\d+)\/(\w+)\/(\d+):(\d+):(\d+):(\d+)/;

    $mon = Decode_Month($mon);

    my $stamp = Date_to_Time($year,$mon,$day,$hour,$min,$sec);

}
[download]

I personally wouldn't bother with all that unneccessary time consuming Date::Calc nonsense, and if you do that, you wouldn't have to bother with the custom compare routine.

On another note, your get_timestamp sub has a classic logic flaw, you assign $1 to something, without ever knowing if there was a match. You should if(/(match)){ $foo = $1; ... }

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