Why does an undef argument in a join produce this warning?

[Anonymous Monk]

Here is my code:

sub test2{

    return "HTML";

}


sub test {

    return join '', test2("1", "$ENV{'HTTP_HOSTS'} ");

}

print test();

And here is the result:

C:\WINDOWS\system32\cmd.exe /c sh
$ perl -w test.PL
Use of uninitialized value in concatenation (.) or string at test.PL l
+ine 11.
HTML$

I am using perl, v5.8.3 built for MSWin32-x86-multi-thread. The second argument passed to test2 is undef, but why would that cause this error? Notice that test2 still returns "HTML". And how best to correct this error?

[wfsp]

The second argument is not undef. The double quotes

"$ENV{'HTTP_HOSTS'} "

says that you are trying to concatanate an undef value with, in this case, a space. That is what Perl is complaining about. Remove the quotes and the result is

HTML

nit: You don't need the quotes around the hash key either.

[Anonymous Monk]

Thanks. you and chromatic were right

[mscharrer]

You already have the solution for this case. If you ever have issues with unwanted warnings again, you should know that there can be specifically deactivated locally in the current scope.

E.g. if you know that one function can cause this warning and the only way around would be massive use of if defined you can just say no warnings 'uninitialized'; to tell Perl that you don't want this specific warning inside the current scope, i.e. sub function or { } block, etc.

[chromatic]

Interpolating a variable containing an uninitialized value into a string produces that warning if you have warnings enabled. Remove the double quotes and the warning will disappear. join has nothing to do with it.

[Anonymous Monk]

But sometimes I need to interpolate that value and pass it to the test2 sub. Hmmm... perhaps I could check if the variable is not defined before passing to the sub.