Well, if you can't install any CPAN modules, you could use a module that comes bundled with perl, namely POSIX. The below example code shows how to create a calendar time for December 12, 1995, at 10:30 am:

$time_t = POSIX::mktime( 0, 30, 10, 12, 11, 95 );
print "Date = ", POSIX::ctime($time_t);
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Presumably you could create 2 calendar times and subtract one from the other using difftime, which will give you the difference in seconds - format as required for display.

UPDATE (small example):

use strict;
use warnings;
use POSIX qw(mktime difftime ctime);

my $previous = '10111623';
my $current = '10111705';

my $time1 = mktime( 0, 0, $4, $3, $2 - 1, $1 + 100 ) if $previous =~ m
+/(\d\d)(\d\d)(\d\d)(\d\d)/;
my $time2 = mktime( 0, 0, $4, $3, $2 - 1, $1 + 100 ) if $current =~ m/
+(\d\d)(\d\d)(\d\d)(\d\d)/;

printf("%s - %s = %d seconds\n", ctime($time2), ctime($time1), difftim
+e($time2, $time1));
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Output:

Wed Nov 17 05:00:00 2010 - Tue Nov 16 23:00:00 2010 = 21600 seconds