Perl: script and module calling a second module - perl

Question

I have the following:
Module1
package module1;
require Exporter;
our #ISA = qw(Exporter);
our #EXPORT = qw(<list of subs within>);
use Module2;
sub M1S1 ()
{
$x = M2S1();
}
Module 2
package module2;
require Exporter;
our #ISA = qw(Exporter);
our #EXPORT = qw(<list of modules within>);
sub M2S1()
{
...
}
sub M2S2()
{
...
}
Script
use Module2;
use Module1;
$y = M1S1();
$z = M2S2();
When the script calls a sub in Module 1 which in turn calls a sub in Module 2, the sub is not found, even though the script can call those subs directly.
I'm not a beginner to Perl by any means, but I've never fully gotten the hang of modules. Our environment has gotten very dependent on module2, so I don't want to make any changes that would require changing all the scripts that use it. Module1 has limited use so I can make changes to it if necessary.

Answer 1

The file name, the name in the package directive, and the name in the use statement must match, and that includes case.
Module1.pm
package Module1;
use Module1;
Or if you had a non-flat namespace,
Foo/Bar.pm
package Foo::Bar;
use Foo::Bar;
Note that you can have similar problems when you have two exporting modules that use each other (directly or indirectly), but that doesn't seem to the case.
$ cat Module1.pm
package Module1;
use strict;
use warnings;
use Exporter qw( import );
our #EXPORT = qw( M1S1 );
use Module2;
sub M1S1 { M2S1() }
1;
$ cat Module2.pm
package Module2;
use strict;
use warnings;
use Exporter qw( import );
our #EXPORT = qw( M2S1 M2S2 );
sub M2S1 { "M2S1" }
sub M2S2 { "M2S2" }
1;
$ cat script.pl
#!/usr/bin/perl
use strict;
use warnings;
use Module2;
use Module1;
print(M1S1(), "\n");
print(M2S2(), "\n");
$ ./script.pl
M2S1
M2S2