G'day vihar,

You can do this very easily with the builtin modules Time::Piece and Time::Seconds.

Here's an example using Sunday, 1-Dec-2013 which, using your figures, should have 2 days subtracted; resulting in Friday, 29-Nov-2013.

#!/usr/bin/env perl -l

use strict;
use warnings;

use Time::Piece;
use Time::Seconds;

my %day2t = qw{Fri 0  Sat 1  Sun 2  Mon 3 Tue 4  Wed 5  Thu 6};

my $test_date = '1-Dec-2013';
my $t_now = Time::Piece->strptime($test_date, '%d-%b-%Y');
my $t_mod = $t_now - ONE_DAY * $day2t{$t_now->day};

print 'Time now:      ', $t_now->strftime;
print 'Time modified: ', $t_mod->strftime;
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Output:

Time now:      Sun, 01 Dec 2013 00:00:00 UTC
Time modified: Fri, 29 Nov 2013 00:00:00 UTC
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Notes:

• I've used abbreviations for the day names. If you need the full names, use the fullday() method instead of the day() method.
• I've reduced your T and T-n values to simple digits. If you need the forms you originally posted, you'll have to add some additional code.