and there may be multiple rows per key (although each row may well have different information for that key)

You cannot have "multiple rows" per key. That means your "key" is not a "key" since it does not uniquely identifies one record and one record only. It also is a strong indication that your data file is not normalized. That is not bad, it just makes life a lot more difficult.

But your problem has in essence nothing to do with keys and database design. It is simply: upon the first occurrence of a certain value in a certain field, do something. Easy as pie!

use Modern::Perl;

my %keys;

while (<DATA>) {
    my ($key, $data1, $data2) = split /,/;
    unless (exists($keys{$key})){
        # do something
        say $key; # just as an example
        $keys{$key} = 1;
    }
}

__DATA__
key1, abd, 123
key2, dhde, 9+6+
key3, dxdc, edazedaz
key1, dea, 564dz
key1, deksizi, 4833
key4, eoz, 852662
key2, dzdadazd, 9566

[download]

Output:

key1
key2
key3
key4

[download]

CountZero

A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

In reply to Re: need for speed - how fast is a hash by CountZero
in thread need for speed - how fast is a hash by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.