In addition to AM's suggestion above, I would look at the possibility or need of supporting multiple versions. Perhaps replace the original module with a find-and-load module, and move the original one to Module::Name::VersionX, where VersionX is the support code for a single, standardized perl API.

... Is there a general guide on how to replace such a filter, done by a module, with a module that uses an external program?

Its easy to create a new Template::Plugin::Filter, you simply copy/paste the existing Template::Plugin::MultiMarkdown, rename it (say Template::Plugin::MultiMarkdownBin ), and replace the module-calling-part with something that calls the external program.

To know how to call this external program, the first step is locating the documentation for this external program to learn how to use it.

Then use something as discussed in perlipc (open2,open3, qx ...

You can use Capture::Tiny, IPC::Run3 ....

But you start with the documentation of this external program...

I can handle the external program. With suitable options it takes input from stdin, writes out to stdout and stderr.

E.g. the following works:

cat foo.md | multimarkdown | wc
[download]

(But why would you want to count html kibbles?)

If it helps, assume that I'm filtering through cut, or sort or sed or awk.

WHICH external program is irrelevant to the discussion.

Indeed, what I think I'm looking for is a generic pipe module that can be renamed and used with a single line edit, or that the command to be run is passed as a parameter.

The part you breeze over is the part I'm having trouble with.

Below is the content of Template/Plugin/MultiMarkdown.pm

It in turn makes reference to Text::MultiMarkdown

#$Id: MultiMarkdown.pm 4103 2009-03-02 20:41:50Z andrew $
package Template::Plugin::MultiMarkdown;
use strict;
use base qw (Template::Plugin::Filter);
use Text::MultiMarkdown;

our $VERSION = 0.03;

sub init {
    my $self = shift;
    $self->{_DYNAMIC} = 1;
    $self->install_filter($self->{_ARGS}->[0] || 'multimarkdown');
    return $self;
}

sub filter {
    my ($self, $text, $args, $config) = @_;
    my $m = Text::MultiMarkdown->new(%{$config || {}});
    return $m->markdown($text);
}

<readmore>
1;

__END__

=head1 NAME

Template::Plugin::MultiMarkdown - TT plugin for Text::MultiMarkdown

=head1 SYNOPSIS

  [% USE MultiMarkdown -%]
  [% FILTER multimarkdown %]
  #Foo
  Bar
  ---
  *Italic* blah blah
  **Bold** foo bar baz
  [%- END %]

=head1 DESCRIPTION

Template::Plugin::MultiMarkdown is a plugin for TT, which will format
your text with MultiMarkdown Style.

=head1 SUBROUTINES/METHODS

There are two methods required by the TT plugin API:

=over 4

=item C<init()>

=item C<filter()>

=back

=head1 VERSION

This man page describes version 0.03.

=head1 SEE ALSO

L<Template>, L<Text::MultiMarkdown>, L<http://fletcherpenney.net/multi
+markdown/>

=head1 AUTHOR

Andrew Ford E<lt>A.Ford@ford-mason.co.ukE<gt> (based on the
L<Template::Plugin::Markdown> TT plugin by Naoya Ito
E<lt>naoya@bloghackers.netE<gt>).

=head1 LICENSE AND COPYRIGHT

Copyright (C) 2009 Andrew Ford

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.

=cut

</readmore>
[download]

So, I copy this to MultiMarkdownBin

It no longer uses Text::Markdown so I can eliminate that.

I need something from one of the 284 different IPC modules.

Which one?

WHAT do I replace in the above code?

I am so far from enlightenment, that I'm waiting between photons.