#! perl

open my $module, '>', 'MyModule.pm';

print $module <<CODE
# MyModule.pm, automatically generated by MyModule.pm.PL
# Do not edit by hand!  All changes will be overwritten
# next time you run MyModule.pm.PL!

# module code goes here...

CODE

# Determine if feature foo is available.  Then:

if ($use_foo) {
  print $module <<IMPLEMENTATION
# Code for implementation with feature foo goes here
# ...
IMPLEMENTATION
}
else {
  print $module <<IMPLEMENTATION
# Code for implementation without feature foo goes here
# ...
IMPLEMENTATION
}

# This is pod that the eventual module will contain, and
# which CPAN will show:

print $module <<DOC;

1;

=head1 NAME

MyModule - a cool module.

=head1 SYNOPSIS

=cut
DOC

# ONLY CPAN (and perldoc MyModule.pm.PL) will show this,
# because it is not being printed to MyModule.pm:

=pod
You are viewing this using CPAN or perldoc MyModule.pm.PL.
When you install this module, only one of the two synopses
will be shown, depending on how you configure the
installation.

=cut

if ($use_foo) {

# This pod only shows on CPAN:
=head2 Synopsis when you use feature foo:

=cut

# This pod shows up in CPAN and in the installed module
# when feature foo is enabled:
  print $module <<DOC
=pod

 # (Here's where you would describe how to use the module
 # with feature foo)
 # ...

=cut
DOC
}
else {

# This pod only shows on CPAN:
=head2 Synopsis when you don't use feature foo:

=cut

# This pod shows up in CPAN and in the installed module
# when feature foo is not enabled:
  print $module <<DOC
=pod

 # (Here's where you would describe how to use the module
 # without feature foo)
 # ...

=cut
DOC

}