teamassociated:

Part of the difficulty appears to be that the file may have some lines you want to discard. Since the $. variable keeps track of the lines read and not the lines wanted, using the modulo operator on it is going to fail. However, the modulo operator idea is a *good* one, you just need to keep track of the lines you want to keep yourself:

my $cnt_keepers=0;
while (my $ln = <$f>) {
    next if ....

    # Now we've got a line we want to keep
    ++$cnt_keepers;
    if ($cnt_keepers % 5 == 0) {
        ... every fifth line processing ...
    }
    ...
}
[download]

Another way you could do it is to simply accumulate the lines you want, and when you have five, process the lot of 'em:

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 = ();
    }
}
[download]

It may be coincidence, but it seems like the first record of each group always has a 'Z' (?record type indicator?) around column 20. If it's not a coincidence, then another way to handle it would be to accumulate records until you find a line with a 'Z' marker, then process all the records you have, and then put the new record into the accumulator.

Update: Fixed a code tag.

...roboticus

When your only tool is a hammer, all problems look like your thumb.