readdir is not my friend. If you use File::Find, you don't have to mess with relative vs. absolute pathnames. You can use $File::Find::name, which is a complete pathname. You don't have to worry about '..' and '.'. Etc. etc. Quite untested:

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 $!;
}
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