#!/usr/bin/perl -w
use strict;
$|++;

sub do_something(*@)
{
  local *HANDLE = shift;
  print HANDLE @_;
}

# set up filehandles for this example
# you'll probably use open()...
local *B = *STDOUT;
local *T = *STDERR;

my %map_char_to_handle = (
  B => *B,
  T => *T,
  # etc.
);  

while(<DATA>)
{
  my $test_for_handle = substr($_, 2, 1);
  defined $map_char_to_handle{$test_for_handle} 
    and do_something($map_char_to_handle{$test_for_handle}, $_);
}

__DATA__
ABC123
CAB234
BTT456
BXC789