I'm not sure I understand your requirements. You're going to read lines, keeping only the first 24 bytes, but you're only interested in the first three characters?

You can get byte offsets in a file you're reading using tell. Here's something that will output offsets along with line contents:

my $offset = tell STDIN;
while ( my $line = <STDIN> ) {
    print "[$offset] $line";
}
continue {
    $offset = tell STDIN;
}
[download]

Sample output (note that newlines count in the offsets):

[0] 12345
[6] 1234567890
[17] a
[19] b
[21] c
[download]

You can pick out the first three characters (or first 24) using substr.

Once you have your data, you can stuff it into a hash. If there's a lot of it, and you want it to persist, use DBM::Deep. I'm thinking this will be especially useful if your three-letter-codes are keys, and you need to store a list of offsets where they're found.

    my $tlc = substr $line, 0, 3;
    push @{ $offsets_for{$tlc} }, $offset;
[download]

Hope this helps.

It really depends upon why you are indexing the file?

And how you are going to use that index?

And, just how large are these files? 3 upper case chars == only 17576 unique values.

And why are you going to keep 24 bytes of each line? If that is enough to satisfy your needs for each line, then why do you need the index. If not, you are going to have to seek the file and read the line to get the rest, so why keep any at all?

Your description: " I am only interested in the first 3 characters." and "keeping only the first unique 24 bytes" is confusing.

And why are you going to keep 24 bytes of each line?

I think he said he was going to save the first 24 bits (3 characters). Which may mean we're not looking at the same version of his post...

Look again at the title of this post, oryour post, or mine to which you replied. Or any of the posts posted before he realised his mistake and corrected both the title and content of the OP.

---

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"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

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