I am not sure what modules are out there to help you. I expect there are some for formatting tables. However, here is a script that seems to produce the required output. I make another hash to associate item ids with years they occur, construct a header and sprintf template then the rest can be done in a sort of Schwartzian Transform.

use strict;
use warnings;

my $rhData = {
   1990 => [1, 2, 3],
   1991 => [2, 3, 5],
   1992 => [1, 2, 7, 9],
   1993 => [3, 7, 8, 13],
   1994 => [1, 4, 8, 12, 17],
   1995 => [2, 3, 4, 6, 7, 10, 12, 13, 14, 20],
   1996 => [1, 4, 5, 8, 9, 12, 14, 16, 17, 19, 20],
   1997 => [1, 2, 5, 6, 7, 10, 12, 14, 16, 17, 20],
   1998 => [4, 5, 6, 7, 10, 11, 13, 14, 15, 18, 20],
   1999 => [3, 5, 6, 7, 8, 9, 11, 12, 16, 19, 20]
   };

my $rhRevLookup = {};
foreach my $year (keys %$rhData)
{
    $rhRevLookup->{$_}->{$year} ++ for @{$rhData->{$year}};
}

my @range = (1995 .. 1999);

my %frequencies = ();
$frequencies{$_} ++
   for map { (@{$rhData->{$_}}) } @range;

my $header = q{ItemID};
$header .= qq{    $_} for @range;
$header .= qq{    Freq.\n};

my $template = q{%-10d};
$template .= q{%-8s} for @range;
$template .= qq{%-5d\n};

print $header;
print
   map  { sprintf $template, @$_ }
   map
   {
       my $raRow = [];
       push @$raRow, $_->[0];
       foreach my $year (@range)
       {
           push @$raRow,
              $rhRevLookup->{$_->[0]}->{$year}
              ? q{=}
              : q{ };
       }
       push @$raRow, $_->[1];
       $raRow;
   }
   sort
   {
       $b->[1] <=> $a->[1]
       ||
       $a->[0] <=> $b->[0]
   }
   map  { [$_, $frequencies{$_}] }
   keys %frequencies;
[download]

Here is the output.

ItemID    1995    1996    1997    1998    1999    Freq.
20        =       =       =       =       =       5    
5                 =       =       =       =       4    
6         =               =       =       =       4    
7         =               =       =       =       4    
12        =       =       =               =       4    
14        =       =       =       =               4    
4         =       =               =               3    
10        =               =       =               3    
16                =       =               =       3    
1                 =       =                       2    
2         =               =                       2    
3         =                               =       2    
8                 =                       =       2    
9                 =                       =       2    
11                                =       =       2    
13        =                       =               2    
17                =       =                       2    
19                =                       =       2    
15                                =               1    
18                                =               1
[download]

I hope this is of use.

Cheers,

JohnGG

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.