Program unexpectedly reports "Use of uninitialized value" - perl

Question

This is with regard to my previous question
Hold Subroutine response and set to variable in Perl
The statement Module::thesub("hello")
worked in Module.pm but fails if I move it to main.pl
main.pl
#!/usr/bin/perl
use strict;
use warnings;
use Module;
Module::thesub("hello");
Module.pm
#!/usr/bin/perl
use strict;
use warnings;
package Module;
sub thesub {
state $stored;
$stored = shift if #_;
return $stored;
}
my $testvar = thesub();
print $testvar;
1;
I get this error
Use of uninitialized value $testvar
which means the variable $testvar has no value.
How can I fix this?

Answer 1

Statements in a module run when the module is loaded, at use Module;. At that time thesub() still wasn't called with an argument and $stored in it is undefined. So that is what $testvar gets when it's assigned and the warning is emitted when it's printed.
The $testvar can be used in main
use strict;
use warnings;
use Module;
Module::thesub("hello");
my $testvar = Module::thesub();
even though I am not sure from the question what the purpose of this is.
Remove the assignment and print of $testvar from the module. Note that you'll also need use feature 'state'; at the beginning of the module, to enable the feature pragma.
A few comments on the module you show
No need for #!/usr/bin/perl, as a module is generally not meant to be run
The package Module; is commonly the first line
While what you have works, consider making symbols from the module available to the calling code, so that it can import them and simply say thesub(). A common way is
package Module;
use warnings;
use strict;
use Exporter qw(import);
our #EXPORT_OK = qw(thesub);
sub thesub { ... }
1;
and in the main
use Module qw(thesub);
thesub("hello");
See Exporter for starters and search SO, where there are great many posts on this.