I like DateTime for all my date/time needs, and there is the extension DateTime::Event::Recurrence. And for more complex recurrences, you can use DateTime::Set. In addition, there is DateTime::Event::ICal.

use warnings;
use strict;
use DateTime;
use DateTime::Event::Recurrence;

my $set = DateTime::Event::Recurrence->monthly(days=>7);
my @list = $set->as_list(
    start => DateTime->new( year=>2018 ),
      end => DateTime->last_day_of_month( year=>2018, month=>12 ) );
print $_->ymd,"\n" for @list;

__END__

2018-01-07
2018-02-07
2018-03-07
2018-04-07
2018-05-07
2018-06-07
2018-07-07
2018-08-07
2018-09-07
2018-10-07
2018-11-07
2018-12-07
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