use File::Find;

find(\&wanted, '/temp_gif');

sub wanted {
   return unless /^\d+\.gif$/i;

   (my $new_name = $File::Find::name) 
      =~ s/(\d+)\.gif$/($1+398) . 'GIF'/ei;  

   rename($File::Find::name, $new_name) or die $!;
}