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.