one way you could accomplish this is by keeping a reference to the parent in each of the section objects so your code becomes something like this

package Module::Main

...
sub init {
  my $self = shift;

  $self->{SEC1} = Module::Section1->new($self);
  $self->{SEC2} = Module::Section2->new($self);
  return 1;
}

package Module::Section1;

sub new {
    my $class = shift;
    my $parent = shift || die; #require a parent..
    my $self = {};

   $self->{PARENT} = $parent;
   bless($self,$class);
   return $self;
}

sub do_something {
    my $self = shift;
   
    $self->{PARENT}->{SEC2}->method();
    return;
}
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update: Or you could keep a reference to the Section2 object which might be a better method OO wise.