Thanks! That code is complete genius and worked first time! I wish I could vote the same posts 10 times :-)

For anyone that wants to try the full program check this out:

#!c:\perl\bin\perl

use strict;
use warnings;

my %category_hash;
my %categories;

# Build a hash containing all of our data records...
while (<DATA>) {
  chomp;
  my %cat;
  @cat{'id','name','parent'} = split /,/;
  $categories{$cat{id}} = \%cat;
}

# ... and turn it into a tree

foreach $_ (keys %categories) {
  if ($categories{$_}{parent}) {
    $categories{$categories{$_}{parent}}{children}{$_} =
      $categories{$_};
  } else {
    $category_hash{$_} = $categories{$_};
  }
}

# And now %category_hash is exactly the same as it was
# in my previous post. Carry on with "show" as before.

#You can then display the structure with code like this: 

show(\%category_hash, 0);

sub show {
  my ($hash, $lvl) = @_;

  my $prefix = '  ' x $lvl;

  foreach (sort keys %$hash) {
    print "$prefix$_ : $hash->{$_}{name}\n";
    show($hash->{$_}{children}, ++$lvl)
      if exists $hash->{$_}{children};
  }
}


__DATA__
1,whatever,0
2,something,1
3,something,1
4,somethingelse,1
5,another subcategory,2
6,something else,0
7,something else,0
[download]

Thanks again davorg

Tom