So rather than generating a date string "Sat Aug 21 22:00:53 2010" and then parsing it with a regex ( not even substr()! .. shocking! ), why not use the day of the week component

How many times do you need to calculate 'last sunday' in any given run of a program?

I think that if you are doing it more than once, you've programmed an error. Even if the run transitions midnight, using two different definitions of that term within a single run of the program, is likely to be an inconsistency with ramifications.

On that basis, you should only calculate it once. And so the performance gain of only taking 3 milliseconds over 6 is neither here nor there.

Even if you do the sensible thing and (having obtained the day of week) perform a calculation rather than a loop:

sub lastSunday2 {
    my $now = time;
    my @now = localtime $now;
    return $now -= ( $now[ 6 ] * 2 - 1 ) * 43200;
}
[download]

And so cut the cost of that single call to 1 millisecond, you're still never going to notice it.