sub process_drop_file {
    my $file = Path::Class::File->new(shift);
    -f $file or croak "'$file' is not a file";
    sleep(1) until (-s $file);                 # <-- New line
    my $doc = XML::LibXML->new->parse_file("$file");
    # Do stuff with it...
    $file->remove or croak "Couldn't remove '$file'";
}