#!/usr/bin/env perl -l

use strict;
use warnings;
use lib '.'; 

use Decoupled qw{
    preprocess_complex_message
    handle_generic_message
};

my %decode_type = (
    A => sub { 
        handle_generic_message(
            A => preprocess_complex_message($_[0])
        )
    },
    B => sub { 
        handle_generic_message(B => $_[0])
    },
);

while (<DATA>) {
    chomp;
    my ($type, $msg) = split /\s+/, $_, 2;
    my $decoded = $decode_type{$type}->($msg);
    print $decoded;
}

__DATA__
B message B1
A more complex message A1
B message B2

##</code><code>##

package Decoupled;

use strict;
use warnings;

use Exporter 'import';
our @EXPORT_OK = qw{
    preprocess_complex_message
    handle_generic_message
};

sub preprocess_complex_message { 
    $_[0] =~ /more complex (.*)/; $1
}

sub handle_generic_message {
    sprintf "Do Type%s stuff with '%s'", @_
}

1;

##</code><code>##

Do TypeB stuff with 'message B1'
Do TypeA stuff with 'message A1'
Do TypeB stuff with 'message B2'