my %hash;
for ( readdir DIR ) # or wherever your filenames come from.
{
  if ( /^([a-z0-9]*?-[a-z]{2}-[a-z]{2,3})-(\d{8})(-eol)?\.(pdf|html)$/
+ )
  {
    push @{ $hash{$1} }, $_;
  }
}
[download]

And there you have it.

Now, if you want to access the keys and filenames in sorted order, you could do --

for my $key ( sort keys %hash )
{
  print "Files for $key:\n";
  for my $file ( sort @{ $hash{$key} } )
  {
    print "\t$file\n";
  }
}
[download]

jdporter
The 6th Rule of Perl Club is -- There is no Rule #6.