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!