how shall I call the functions from the main? $m->half(10); ? This doesn´t work, since the output is Mymodule=HASH(0xbd5068) and not just 5! What am I doing wrong?

If you're doing it the $m->half() way, then you need to have a module that looks like this:

package MyPackage;

sub new {
   my ($class) = shift();
   my $self = {};
   return bless $self, $class;
}

sub half {
   my $self   = shift();
   my $number = shift();
   return $number / 2;
}

1;
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and a script that looks like this:

use MyPackage;
my $m = MyPackage->new();
print $m->half(10);
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Where you create a new MyPackage object and then use the object->function call to get the result.