It's important to be clear on definitions and provide meaningful examples. For example, sometimes a "month" is defined as exactly 30 days, but since you're asking this question, I'm guessing that's not what you want. For example, using DateTime math:

2016-01-29 + 30 days = 2016-02-28    2017-01-29 + 30 days = 2017-02-28
2016-01-30 + 30 days = 2016-02-29    2017-01-30 + 30 days = 2017-03-01
vs.
2016-01-29 + 1 month = 2016-02-29    2017-01-29 + 1 month = 2017-03-01
2016-01-30 + 1 month = 2016-03-01    2017-01-30 + 1 month = 2017-03-02

I would recommend this: First, calculate the "deadline" date by adding your "6 months" to the $date_birth. Then, use the methods provided by the library to see if the $diab_confirm is past that date or not. For example, in Date::Calc that would probably be Add_Delta_YM for the first part, and Date_to_Days or Delta_Days for the second.

Personally, I like DateTime, and its objects overload the comparison operators:

use warnings;
use strict;
use DateTime;
use DateTime::Format::Strptime;

my $date_birth = '21/02/2013';
my $diab_confirm = '29/08/2013';

my $strp = DateTime::Format::Strptime->new(on_error=>'croak',
    pattern => '%d/%m/%Y');

my $dt_birth = $strp->parse_datetime($date_birth);
my $dt_confirm = $strp->parse_datetime($diab_confirm);
print "   birth: ", $dt_birth->ymd, "\n";
print " confirm: ", $dt_confirm->ymd, "\n";

my $dt_deadline  = $dt_birth->clone->add(months=>6);
print "deadline: ", $dt_deadline->ymd, "\n";

if ($dt_confirm > $dt_deadline) {
    print $dt_confirm->ymd, " > ", $dt_deadline->ymd, "\n";
}
else {
    print $dt_confirm->ymd, " <= ", $dt_deadline->ymd, "\n";
}

__END__

   birth: 2013-02-21
 confirm: 2013-08-29
deadline: 2013-08-21
2013-08-29 > 2013-08-21
[download]

As always, the more test cases the better!

Room for one more? Here's a generic Time::Piece solution:

use strict;
use warnings;
use Time::Piece;
use Test::More;

my $fmt     = '%d/%m/%Y';
my $date    = Time::Piece->strptime ('21/02/2013', $fmt);
my @within  = ('12/08/2013');
my @without = ('29/08/2013');
my $mon     = 6;

plan tests => @within + @without;

for my $other (@within) {
    ok (within_months ($mon, $date, Time::Piece->strptime ($other, $fm
+t)),
        "$other is within $mon months of $date");
}

for my $other (@without) {
    ok (! within_months ($mon, $date, Time::Piece->strptime ($other, $
+fmt)),
        "$other is not within $mon months of $date");
}

sub within_months {
    my ($mon, @d) = @_;
    if ($d[0] > $d[1]) { push @d, shift @d };
    my $start = $d[0]->add_months ($mon);
    return $start > $d[1];
}
[download]

Feel free to expand the @within and @without arrays with more test cases. You can also change $mon to whatever number you require. Time::Piece is in core so there are no extra dependencies needed here.

Here is one way:

use strict;
use warnings;
use DateTime;

my $dt1 = DateTime->new
(
    year  => 2013,
    month =>    9,
    day   =>   29,
);
 
my $dt2 = DateTime->new
(
    year  => 2013,
    month =>    3,
    day   =>   29,
);
 
my $dt3 = DateTime->new
(
    year  => 2013,
    month =>    3,
    day   =>   30,
);

my $dur1 = $dt2->subtract_datetime($dt1);
printf "29/03/2013 to 29/09/2013: %s months\n", $dur1->months;

my $dur2 = $dt3->subtract_datetime($dt1);
printf "30/03/2013 to 29/09/2013: %s months\n", $dur2->months;
[download]

Output:

17:44 >perl 1919_SoPW.pl
29/03/2013 to 29/09/2013: 6 months
30/03/2013 to 29/09/2013: 5 months

17:44 >
[download]

Update: Please see the correction by haukex, below.

Hope that helps,

Unfortunately, one has to be careful with DateTime::Duration objects' accessors: ->month only returns the "months" component of the difference. You usually want ->in_units instead, but note the limitations of conversion - for example, you can't convert a number of months to a number of days without knowing which months we're talking about (and that information is no longer available if we only have a duration), so you have to get both the months and days, and in_units will do the conversion from years to months for you.

use warnings;
use strict;
use DateTime;

my $dt1 = DateTime->new(year => 2013, month => 10, day =>  1);
my $dt2 = DateTime->new(year => 2018, month =>  3, day => 31);

my $dur1 = $dt2->subtract_datetime($dt1);
print $dt1->ymd, " to ", $dt2->ymd, ": ",
    $dur1->months, " months - oops!\n";

my ($days,$months) = $dur1->in_units('days','months');
print $dt1->ymd, " to ", $dt2->ymd, ": ",
    "$months months, $days days", "\n";

__END__

2013-10-01 to 2018-03-31: 5 months - oops!
2013-10-01 to 2018-03-31: 53 months, 30 days
[download]

That is great!
Thank you so much!