Done it! Of course, you did give me just a teensy-weensy little hint, Chmrr... many thanks! Here's the final (?) version:

#!/usr/bin/perl
use warnings;
use strict;
use CGI;

## Chmrr's revision, very slightly modified

## Column headings
my @members = qw/ Algeria Indonesia Iran Iraq Kuwait Libya Nigeria Qat
+ar Saudi_Arabia UAE Venezuela total_OPEC /;
@members = map { tr/_/ /; $_ } @members;

## Load data
my (%quotas, @dates);
while (<DATA>) {
    ( my $period, $_ ) = split ':';
    push @dates, $period;
    @{$quotas{$period}}{@members} = split ' ';
}

## Introductory paragraph
my $query = CGI->new();
print $query->header("text/html"),
  $query->start_html( -title   => "QuotaBase: a database of OPEC oil p
+roduction quotas",
                      -bgcolor => "#cbcbcb" ),
  $query->h1("Welcome to QuotaBase!"),
  $query->table(
                $query->Tr(
                           $query->td({-width => "600"},
                                      $query->p(<<EOP)
QuotaBase is an interactive database of OPEC oil production quotas. By
+ default, the <b>current quotas</b> are displayed. To view <b>histori
+cal quota information</b>, select the period you want from the drop-d
+own list and click the 'Show quotas' button. A table of all the quota
+s for that period will be  displayed.
EOP
                                     )
                          )
               ),
  $query->start_form(),
  "Choose a period: ",
  "&nbsp;",
  $query->popup_menu( -name=>'period',
                      -values=>[@dates],
                      -default=>(@dates)[0]),
  "&nbsp;&nbsp;",
  $query->submit(-name=>'submit', -value=>'Show quotas'),
  "&nbsp;&nbsp;",
  $query->defaults('Reset current quotas'),
  $query->endform;

## Data table
my $period = $query->param('period') || (@dates)[0];
print $query->table({-border=>1},
                    $query->Tr(
                               $query->td({-colspan=>2,-align=>"center
+"},"<b>$period</b>")
                              ),
                    map {
                      $query->Tr(
                                 $query->td({-width=>130,-align=>"left
+" }, $_),
                                 $query->td({-width=>130,-align=>"righ
+t"}, $quotas{$period}{$_})
                                )
                    } sort keys %{$quotas{$period}}
                   );
print $query->end_html;


__DATA__
Jan 02 - Dec 02:        693 1125 3186 0 1741 1162 1787 562 7053 1894 2
+497 21700
Sep 01 - Dec 01:        741 1203 3406 0 1861 1242 1911 601 7541 2025 2
+670 23201
Apr 01 - Aug 01:        773 1255 3552 0 1941 1296 1993 627 7865 2113 2
+786 24201
Feb 01 - Mar 01:        805 1307 3698 0 2021 1350 2075 653 8189 2201 2
+902 25201
31 Oct 00 - Jan 01:     853 1385 3917 0 2141 1431 2198 692 8674 2333 3
+077 26700
1 Oct 00 - 30 Oct 00:   837 1359 3844 0 2101 1404 2157 679 8512 2289 3
+019 26200
[download]