Some minor suggestions:

1. In GetFileSize, change this:

           if ($res->is_success) {
              my $headers = $res->headers;
              return $headers;
           }else{
              $flag = 1;
              $error .= "Error retrieving file information at $url ";
           }
           return 0;
   [download]

   To this:

           return $res->headers    if $res->is_success;
   
           $flag   = 1;
           $error .= "Error ...";
           return 0;
   [download]

2. Within your for loop, change this:

      if(exists($extens{$ext})){
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   To this, and remove one level of indentation.

      next unless exists $extens{$ext};
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3. Change delimiters rather than escaping. And in this example the global modifier isn't necessary.

      #if($newurl !~ /http::\/\/$/ig){
       if($newurl !~ m#http:://$#i){
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4. $skips -= 1; vs. $skips--;?
5. Change this:

   print"\n\n\n$filename at $newurl\n";
   print "File size: ".$header->content_length." bytes\n" unless $flag==1
   +;
   print "Last modified: ".localtime($header->last_modified)."\n" unless 
   +$flag==1;
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   To this:

   print "\n\n\n$filename at $newurl\n",
   printf("File size: %d bytes\nLast modified: %s\n", $header->content_le
   +ngth, localtime($header->last_modified)) unless $flag == 1;
   [download]

6. Spelling and grammar...

      #print"\nExtensions to download (seperated by comma's): ";
       print"\nExtensions to download (separated by commas): ";
   [download]

   Comment on Re: RFC: Code testers/reviewers needed Select or Download Code