You're getting a bit mixed up with all the sigils (%@$). First, there's no need to do this:

my %dupes = (); # name->count
my $dref = \%dupes;
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You can, but there is no need. And in this case you're not passing the hash around, so I've left it as a hash, rather than a hash-ref.

Where you DO need a ref is where you store the filedata in %fileinfo. You want an array containing all the files with name X, and you store an array as a hash value by using an array ref.

  @file_info = qw(data1 data2 data3);
  %files = (
     X  => \@file_info
  )
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If you want to add something to the array, then you need to use push, which means you need an array, not an array ref, so you need to dereference it:

  $file_info_ref = $files{X};
  push @$file_info_ref,'data5';

  or

  push @{$files{X}}, 'data5'
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So:

my %fileinfo;
my %dupes;

# for each file, recursing through directory tree
foreach {
   # here $_ is each file name

   # if filename seen already 
   if( exists $fileinfo{$_} ) { 
       # then record in %dupes
       $dupes{$_} = 1;    
   }

   @filedata = ( $fpath, $fsize, $fdate ); # but using real data

   # if fileinfo doesn't yet have an entry for this $_, 
   # then assign an empty array ref
   # saved_info is now an array ref which is 
   # also stored in $fileinfo{$_}

    $saved_info = $fileinfo{$_} ||= [];
    push @$saved_info,\@filedata;

   # No need to re-store it in the hash, because $fileinfo{$_} 
   # and $saved_info both point to the same array
}
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