my @files;
if (-d $x_file) {
   # $x_file is a directory, not a file
   @files = glob "$x_file/*xml";
}
elsif ($x_file =~ /xml$/ and -e $x_file){
   # $x_file is an xml-file, and we have access to it
   push @files, $x_file;
}
else {
   die("can't make sense out xml-directory or xml-file $x_file");
}