If all you want is the filename extension, try a regular expression pattern with capturing group:

if ($pubupload =~ m![^.]\.([^./\\]+)$!) {
  $extension = $1;
}
[download]

This pattern will match at least one non-dot character, a dot, then capture one or more non . / and \ characters until the end of string.

$pubupload = 'C:\Documents and Settings\Administrator\Desktop\china.tx
+t';
DB<1> if ($pubupload =~ m![^.]\.([^./\\]+)$!) { $extension = $1; }
DB<2> print "[$extension]"                                 
[txt]
[download]

Update:

Modified pattern so that it won't be fooled by dot files (filenames with leading dot) like .notanextension and will require at least one non-dot character in the filename, for example: a.txt.