The != operator performs a numeric comparison. Perl has built-in magic that gives scalars both numeric and string interpretations. It does the best it can at providing useful defaults when a veriable doesn't have a useful numeric equivalent.

'Hi' defaults to the number zero.

undef also defaults to zero.

When you use != against undef you are doing a numeric comparison against zero. Of your variables, only 1 is not numerically equal to zero.

You could try string comparison, eq, but that won't solve your problem either. As a string undef is equivalent to the empty string.

Using defined does exactly what you want.

Using defined also indicates that you intentionally dealing with the case where a variable might not be initialized. Uninitialized variables are an endless source of bugs. That us why you should have warnings turned on. Similarly, use strict; helps prevent a lot of other bugs.

Your program spits out warnings, if warnings are turned on

#!/usr/bin/perl
use warnings;

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"}
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Output:

Use of uninitialized value in numeric ne (!=) at ./delme.perl line 7.
Argument "Hi" isn't numeric in numeric ne (!=) at ./delme.perl line 7.
Use of uninitialized value in numeric ne (!=) at ./delme.perl line 8.
Argument "" isn't numeric in numeric ne (!=) at ./delme.perl line 8.
Use of uninitialized value in numeric ne (!=) at ./delme.perl line 9.
$number is defined
Use of uninitialized value in numeric ne (!=) at ./delme.perl line 10.
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