I came across some unexpected behaviour and I'd be very grateful for any insight as to why this happens. The odd behaviour is illustrated by the following minimal case: 
use strict;
use warnings;

my $tainted = shift;
my $test;

$test = undef;
$test += $tainted;
print "Test 1: $test\n";

$test = undef;
$test += $tainted;
print "Test 2: $test\n";

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This must be run with taint checking on in order to see the issue, so for this example, save as test.pl and run as follows with a numeric argument passed in, eg: 
perl -T test.pl 1

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Now, as I understand it, a '+=' op is allowed on an undefined value, so I'd expect this to work fine and to leave $test with a value of 1 in each case. However, the surprising thing is, this triggers a warning because $test is undefined, but ONLY the second time round after it has first been set with a tainted value and then undefined. So here's what happens: 
%perl -T test.pl 1
Test 1: 1
Use of uninitialized value in addition (+) at test.pl line 12.
Test 2: 1

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I tested this on 2 different Perls, both with same result: It's easy to workaround (eg. don't undef the variable, or if it needs resetting, set it to zero) but I am curious as to why this happens? Anybody got a good explanation for this?

In reply to Inconsistent warnings over undefined values by oxone

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