Allow me to comment a little bit on your code, you are sometimes making your life harder than it has to be ;-) I will make the comments sequentially with the source code.

• For multiline strings use a HERE document like this

  my $keyword = $ARGV[0] || die << "USAGE";
  
  use: agenda keyword (databasefile separator pos.key)
  
    default database:  $db_file
    default separator: $separator
    default pos.key:   $pri
  
  and so on
  USAGE
  
  # the line above marks the end
  [download]

• It is sufficient to use strict only once at the top of your script, you don't have to repeat it in every subroutine.

• open DB, "<$db_file" or die "Can't open $db_file: $!\n";
  
  # leave out the final \n
  open DB, "<$db_file" or die "Can't open $db_file: $!";
  [download]

  Then perl appends automatically the line number where the error occured - very helpful for debugging. And don't worry, perl will put a newline at the end of the statement.
• Use less comments!! Yes, too many comments can be as bad as too little comments as the code gets cluttered and makes reading more difficult, e.g.

  ### Close database
      close DB;
  [download]

  is really unnecessary, both lines read exactly the same. There has been quite some discussion about commenting correctly here at PM, have a look with Super Search.
• This can be contracted

  my $first_record = <DB>;
  chomp $first_record;
  
  # in one line
  chomp(my $first_record = <DB>);
  [download]

• You are throwing the last field of the every entry away, $#names contains the last index of the array @names, not the number of elements. To avoid such one-off errors don't use loops which use indices - if avoidable. Here you can write this instead of the loop: @{ $values{$fields[$primary]} } = @fields;
• Use compiled regexes with the /o modifier if the variable you are interpolating doesn't change, e.g. $item =~ /$keyword/io;
• Don't use printf unless necessary, in your cases you can simply use the string interpolation and write

  print "\n ** keyword: $item\n\n";
  
  # and
  print "    $names[$features] = $values{$item}[$features]\n";
  [download]

There are more things, e.g. I don't think the hash of arrays is the best datastructure for this task, you are not using the main feature of a hash, the direct access to an element by a key.

It has probably been some time since you wrote this script, your perl has improved in the meantime. So as a good exercise I'd suggest to rewrite the whole thing, rethinking your data structure and incorporating my advices. If you then post your script here you will get further suggestions for improvement. And at the end of the day you will have learnt a lot :-)

-- Hofmator