Verifying that a file opened

• When you use this syntax:

open(DATA, ">>../userdata.txt") || &error;
• A subroutine called error, which you define, is called if the file open fails.
• You might want error to generate an HTML page containing an error message.


sub error {
    #   Write the HEAD section, including the title.
    print $query->start_html("Error");
    print "File open failed.";
    print $query->end_html;
}