#!/usr/bin/perl -w

use strict;
use Time::Local;

# Get the epoch time and the day of the week
my $now = time;
my $dow = (localtime($now))[6];

# Get the various elements of the current date and time,
# subtracting the correct number of seconds to move to
# the previous Sunday.
# N.B. 86_400 is the number of seconds in a day
my @time = localtime($now - ($dow * 86_400));

# Move to the previous midnight
@time[0 .. 2] = (0, 0, 0);

# Get the epoch seconds. This is now the
# start of the previous Sunday
my $sunday = timelocal(@time[0 .. 5]);

# Add a day to get to the start of the previous
# Monday
my $monday = $sunday + 86_400;

# At this point you can do what you want with this
# number  - maybe use it in the where clause of
# a database query.
print scalar localtime $monday;