The Undefined Value

• The undef value.
• Used as a number, undef converts to zero.
• Used as a string, undef converts to the null string.

Where else could undef come from?

• Functions
• Forms

if ( !defined($dbh) ) {
    die "Could not connect to db.\n";     }

undef isn't bad, but...

• When Perl is invoked with the -w flag, undef generates warnings that disrupt the flow of HTML to the browser.
• The DBI::connect subroutine could return undef in place of an object.
• Using undef as an object causes Perl to terminate.

Review

Why does Perl emit a warning when undef is used?