I want to know the reason why I have to use defined when I need to know whether a variable is defined instead of using direct comparison with undef.
For example this code isn't work as I expected

#!/usr/bin/env perl

my($str,$empty_str,$number,$number_zero)=('Hi','',1,0);


if($str!=undef){print"\$str is defined\n"}  
if($empty_str!=undef){print"\$empty_str is defined\n"}
if($number!=undef){print"\$number is defined\n"}
if($number_zero!=undef){print"\$number_zero is defined\n"}

[download]

output: $number is defined

In reply to why can't compare a variable with undef directly? by Anonymous Monk

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