my %dupes = (); # name->count
my $dref = \%dupes;

##</code><code>##

  @file_info = qw(data1 data2 data3);
  %files = (
     X  => \@file_info
  )

##</code><code>##

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

  or

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

##</code><code>##

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
}