my %entries; # key=calendar name; value=array(ref) of dates.
    my $calendar;
    my @dates;

        sub finish_pending_entry {
            if ( defined $calendar ) {
                $entries{$calendar} = [ @dates ];
            }
            # re-init the state data:
            $calendar = undef;
            @dates = ();
        }

    while (<>) {
        chomp;
        if ( /calendar: (.*)/ ) {
            finish_pending_entry(); # possibly none, if this is the first line.
            $calendar = $1;
        }
        else {
            push @dates, $_;
        }
    }
    finish_pending_entry(); # possibly none, if file was empty.