my @records;
while (my $ln = <$f>) {
    next if ...

    push @records, $ln;

    if (@records==5) {
        print "A $records[0]\n";
        print "B $records[1]\n";
        print "C $records[2]\n";
        print "D $records[3]\n";
        print "E $records[4]\n";
        print "F $records[5]\n";
        @records = ();
    }
}