if you stick with this solution, i'd use elsif instead of else if--it's proper perl. it's already been mentioned to cache the result from substr, so i'll leave that alone.

but i'd be inclined to use a hash rather than elseif. if you use subroutines for the work, it'll be easy to maintain your code when you need to do more than just print to a handle, because all the code's in one place. here's a fully working example:

#!/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
[download]

~Particle *accelerates*