/home/user/perl_modules/lib/perl5/My/Foo.pm:

package Foo;

our $VERSION = '1.0';

use strict;
use warnings;

sub new
{
    my $class = shift();

    my ($arguments) = @_;

    my $self = {};

    bless $self, $class;
}
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foo_version_test.pl:

#!/tool/bin/perl -w
use strict;
use lib '/home/user/perl_modules/lib/perl5';
use My::Foo 1.0;

printf("version = %s\n", $Foo::VERSION);
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Filename and package name do not match.

If the file name (relative to the include path) is My::Foo, and you load the module in foo_version_test.pl by use My::Foo, then the package name should be My::Foo, not Foo. Loading the module creates a global variable named $Foo::VERSION, but use My::Foo 1.0 tests for a variable named $My::Foo::VERSION.

It is perfectly legal to have package names in a module that to not match the file name, so perl does not warn. But it causes a lot of trouble (see Breaking Tie::Hash into three modules for an example). As a rule of thumb, make sure that package names and file names match.

Update: Modern versions of perl also allow package My::Foo 1.0;, implicitly setting $My::Foo::VERSION and matching use My::Foo 1.0. See also package.

Alexander