I dare come again out of the wood...

It seems that it is important that a DateTime::Set has both start and end point.

Here is my today's attempt with and without an end point and the output below is different (correct if end point is defined):

#!/usr/bin/perl
use strict;
use warnings;

use DateTime;
use DateTime::Set;

my $start = DateTime->today();
my $end   = DateTime->today()->add('days' => 90); 

my $biweekly = DateTime::Set->from_recurrence(
    'recurrence' => sub {
          return $_[0]->add('days' => 14)->truncate('to' => 'day')
        },
     'start' => $start,
     'end' => $end, # !!! This is critical.
);

my $iter = $biweekly->iterator;
print "This is the set with defined 'end': ", $/;
while ( my $dt = $iter->next )
{
    print $dt->datetime, $/;
};
print $/;

print 'TESTING whether Date is in set...', $/;
my $date = DateTime->today();
print $biweekly->contains($date), ' ', $date->ymd, $/;
$date->add('days' => 7);
print $biweekly->contains($date), ' ', $date->ymd, $/;
$date->add('days' => 7);
print $biweekly->contains($date), ' ', $date->ymd, $/;
$date->add('days' => 7);
print $biweekly->contains($date), ' ', $date->ymd, $/;
$date->add('days' => 7);
print $biweekly->contains($date), ' ', $date->ymd, $/;
$date->add('days' => 7);
print $biweekly->contains($date), ' ', $date->ymd, $/;

print $/;
print "This is the begin of the set with 'end' commented out: ", $/;

$biweekly = DateTime::Set->from_recurrence(
    'recurrence' => sub {
          return $_[0]->add('days' => 14)->truncate('to' => 'day')
        },
     'start' => $start,
     # 'end' => $end, # !!! This is critical.
);

$iter = $biweekly->iterator;
while ( my $dt = $iter->next )
{
        last if $dt > $end;  # to avoid the infinite loop.
        print $dt->datetime, $/;
};
print $/;

print 'TESTING whether Date is in set...', $/;
$date = DateTime->today();
print $biweekly->contains($date), ' ', $date->ymd, $/;
$date->add('days' => 7);
print $biweekly->contains($date), ' ', $date->ymd, $/;
$date->add('days' => 7);
print $biweekly->contains($date), ' ', $date->ymd, $/;
$date->add('days' => 7);
print $biweekly->contains($date), ' ', $date->ymd, $/;
$date->add('days' => 7);
print $biweekly->contains($date), ' ', $date->ymd, $/;
$date->add('days' => 7);
print $biweekly->contains($date), ' ', $date->ymd, $/;
[download]

The output:

C:\Perl\bin>perl N:\Perle\Learn\DateTime\pm_1033231_orig_analyse_003_h
+.pl
This is the set with defined 'end':
2013-05-14T00:00:00
2013-05-28T00:00:00
2013-06-11T00:00:00
2013-06-25T00:00:00
2013-07-09T00:00:00
2013-07-23T00:00:00
2013-08-06T00:00:00

TESTING whether Date is in set...
1 2013-05-14
0 2013-05-21
1 2013-05-28
0 2013-06-04
1 2013-06-11
0 2013-06-18

This is the begin of the set with 'end' commented out:
2013-05-14T00:00:00
2013-05-28T00:00:00
2013-06-11T00:00:00
2013-06-25T00:00:00
2013-07-09T00:00:00
2013-07-23T00:00:00
2013-08-06T00:00:00

TESTING whether Date is in set...
1 2013-05-14
1 2013-05-21
1 2013-05-28
1 2013-06-04
1 2013-06-11
1 2013-06-18
[download]