#!/usr/bin/env perl -l

use strict;
use warnings;

my %decode_type = (
    A => sub { "Do TypeA stuff with '$_[0]'" },
    B => sub { "Do TypeB stuff with '$_[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>##

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