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  1. or download this
    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!";
}
  2. or download this
    if (undef ne "") {#string context
   print "pg is wrong!";
} else {
   print "pg is right!";
}
  3. or download this
    my $a;