# lib/Animal.pm

package Animal;

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

sub is_furry {
  my $self = shift;
  warn "We don't know if this animal is furry or not!\n";
  return undef;
}

sub make_noise {
  my ( $self, $noise ) = @_;
  print $noise, "\n";
}

1;

# lib/Bear.pm

package Bear;

use base 'Animal';

# We know bears are furry, so override Animal's is_furry method:
sub is_furry {
  my $self = shift;
  return 1;
}

sub eat_honey {
  my $self = shift;
  # Make use of Animal's make_noise method:
  $self->make_noise( "Yum!" );
}

1;

# example.pl

use Animal;
use Bear;

my $baloo = Bear->new;
if ( $baloo->is_furry ) {
  $baloo->eat_honey;
}

##</code><code>##

# AliceUtils.pm

package AliceUtils;

sub safe_filename {
  my $string = shift;
  $string =~ s{\W}{}g;
  $string = "untitled" if !$string;
  return $string;
}

1;

# BobUtils.pm

package BobUtils;

use AliceUtils;  # Make sure it's loaded!

sub save_data {
  my ( $filename, $data ) = @_;
  my $safe_filename = AliceUtils::safe_filename( $filename ) . ".txt";
  open my $fh, '>', $safe_filename or die;
  print $fh $data or die;
  close $fh or die;
  return $safe_filename;
}

1;

# example.pl

use AliceUtils;
use BobUtils;

BobUtils::save_data( "my file", "Hello world!\n" );

##</code><code>##

# AliceUtils.pm

package AliceUtils;

# Tell Exporter::Shiny which functions to make available to others:
use Exporter::Shiny 'safe_filename';

sub safe_filename {
  my $string = shift;
  $string =~ s{\W}{}g;
  $string = "untitled" if !$string;
  return $string;
}

1;

# BobUtils.pm

package BobUtils;

# Tell AliceUtils which functions we want from it:
use AliceUtils 'safe_filename';

# Tell Exporter::Shiny which functions to make available to others:
use Exporter::Shiny 'save_data';

sub save_data {
  my ( $filename, $data ) = @_;
  my $safe_filename = safe_filename( $filename ) . ".txt";
  open my $fh, '>', $safe_filename or die;
  print $fh $data or die;
  close $fh or die;
  return $safe_filename;
}

1;

# example.pl

use AliceUtils;
use BobUtils 'save_data';

save_data( "my file", "Hello world!\n" );