Given:

use warnings;
use strict;

package Frobnicate::Bar;

sub new {
    my ($class) = @_;
    return bless {}, $class;
}


sub sayWhat {
    my ($self) = @_;
    
    if (ref $self) {
        print "I'm a ", ref $self, " method\n";
    } else {
        print "I'm a ", __PACKAGE__, " function\n";
    }
}


package Frobnicate::Foo;

our @ISA = qw/Frobnicate/;

sub new {
    my ($class) = @_;
    return Frobnicate::new ($class);
}


sub sayWhat {
    my ($self) = @_;
    
    if (ref $self) {
        print "I'm a ", ref $self, " method\n";
    } else {
        print "I'm a ", __PACKAGE__, " function\n";
    }
}


package Frobnicate;


sub new {
    my ($class) = @_;
    return bless {}, $class;
}


sub sayWhat {
    my ($self) = @_;
    
    if (ref $self) {
        print "I'm a ", ref $self, " method\n";
    } else {
        print "I'm a ", __PACKAGE__, " function\n";
    }
}


1;
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consider:

use strict;
use warnings;
use noname1;

Frobnicate::Bar->new ()->sayWhat ();
Frobnicate::Foo->new ()->sayWhat ();
Frobnicate->new ()->sayWhat ();
Frobnicate::Bar::sayWhat ();
Frobnicate::Foo::sayWhat ();
Frobnicate::sayWhat ();
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which prints:

I'm a Frobnicate::Bar method
I'm a Frobnicate::Foo method
I'm a Frobnicate method
I'm a Frobnicate::Bar function
I'm a Frobnicate::Foo function
I'm a Frobnicate function
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So, exactly what is hidden from the calling code exactly?

btw, I don't condone the use of the same sub as a method and a function. Used here to reduce code peripheral to the main thesis.

Incorrect. Any external code which does a use Frobnicate will get access to all packages/classes within Frobnicate.pm. The relationship of "file name == package name" is purely a matter of convention. It is not enforced in any way by perl.

Let's create a module Foo.pm containing:

package Bar;

sub identify {
  print "I'm in package " . __PACKAGE__ . "\n";
}

1;
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#!/usr/bin/perl 

use Foo;

Bar->identify();
Foo->identify();
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I'm in package Bar
Can't locate object method "identify" via package "Foo" (perhaps you f
+orgot to load "Foo"?) at...
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But don't do that in production code, of course. The reason for the convention is because use Foo giving you access to package Bar is confusing as hell, especially if it doesn't give access to package Foo.