if (undef != 0) {#in number context, undef evaluates to 0, and we know "0 != 0" does not stand.
   print "pg is wrong!";
} else {
   print "pg is right!";
}

##</code><code>##

if (undef ne "") {#string context
   print "pg is wrong!";
} else {
   print "pg is right!";
}

##</code><code>##

my $a;