Sounds like you want DateTime's ->truncate method:

use warnings;
use strict;
use DateTime;

my $now = DateTime->new(year=>2020,month=>4,day=>21,hour=>12);
my @tests = (
    DateTime->new(year=>2020,month=>4,day=>27,hour=>13),
    DateTime->new(year=>2020,month=>4,day=>28,hour=>11),
    DateTime->new(year=>2020,month=>4,day=>28,hour=>13),
    DateTime->new(year=>2020,month=>4,day=>29,hour=>11)
);

for my $reservation_date (@tests) {
    my $fc_days_limit=7;
    if ($fc_days_limit) {
        my $days_from_now = $now->clone->add( days=>$fc_days_limit )
            ->truncate( to => 'day' );
        my $res_date_only = $reservation_date->clone
            ->truncate( to => 'day' );
        if ($res_date_only >= $days_from_now) {
            print "Error on $reservation_date\n";
        }
        else {
            print "Reservation $reservation_date is ok\n";
        }
    }
}

__END__

Reservation 2020-04-27T13:00:00 is ok
Error on 2020-04-28T11:00:00
Error on 2020-04-28T13:00:00
Error on 2020-04-29T11:00:00
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