#如果给定的值不是undef, defined()函数会返回 true #如果给定值是undef 则返回false.my $test01 = "" ;#it return ture it will print string.if (defined $test01){    print "var is defined\n";}#it return false it will print string.if (! defined $test02){    print "var is undef\n";}