Here's my version (hash of hash): 
#!/usr/bin/perl
use strict;

my %data_by_employee;
my %data_by_form;

# read in tab-delimited data fields into
# two hashes-of-hashes (actually, two hashes of hash refs)
while(my $line = <DATA>) {
  chomp $line;
  my ($employee, $form, $date) = split(/\t/,$line);
  next unless ($employee && $form && $date); # skips blank lines

  $data_by_employee{$employee}{$form} = $date;
  $data_by_form{$form}{$employee} = $date;
}

print "By Employee:\n";
for my $employee (sort keys %data_by_employee) {
  print "$employee\n";
  # note: $data_by_employee{$employee} is a hash reference,
  # so we have to dereference it by using %{ }
  for my $form (sort keys %{ $data_by_employee{$employee} } ) {
    my $date = $data_by_employee{$employee}{$form};
    print "\t$form\t$date\n";
  }
}
print "\n";

print "By Form:\n";
for my $form (sort keys %data_by_form) {
  print "$form\n";
  # note: $data_by_form{$form} is a hash reference,
  # so we have to dereference it by using %{ }
  for my $employee (sort keys %{ $data_by_form{$form} } ) {
    my $date = $data_by_form{$form}{$employee};
    print "\t$employee\t$date\n";
  }
}
print "\n";

__DATA__
10001   10      20090101
10002   10      20080515
10003   10      20090323
10001   20      20090412
10002   20      20090711

[download]

In reply to Re: Data Structure Question by scorpio17
in thread Data Structure Question by bohrme

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.