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#!/usr/bin/perl
use warnings;
...
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"}

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 9.
$number is defined
Use of uninitialized value in numeric ne (!=) at ./delme.perl line 10.