The problem is the foreach passes to block just the filename (1.gif, 3.gif). You should concatenate the path (/temp_gif) and the filename (3.gif) when rename()ing.

$dir = "/temp_gif";
opendir(DH, $dir) or die $!;
foreach (readdir DH) {
   next if /^\.?\.$/;
   ($number, $gif) = split /\./;
   print "$number\n";
   $numbers_replace = int($number) + 338;
   rename("$dir/$_", "$dir/$numbers_replace.GIF") or warn $!;
}
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