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

##
# NOTE:  This code Assumes (and we all know what that means) that the file
# being fed to has no more than one "boundary" (/FH|BH/) per line, and 
# that the file is delimited by newlines.
#

use Switch 'Perl6'; # Import thedamian's sugar, it's better than C&H.

use English '-no_match_vars'; # since we're using some Perl 6 syntax here, may
                              # as well get rid of $0 in the usage statement


my $file = shift or
  die "USAGE $PROGRAM_NAME filename\n";

open( FH, $file ) or
  die "Coudln't open $file: $!\n";

my $batchnum = 0; # global batch tracker

while (<FH>) {
  chomp();
  next if m/^$/;
  given ($_) {
    when /^FH$/   { print "File Header\n"; last; }
    when /^BH$/   { print "Batch Header\n"; $batchnum++; last; }
    when /^FT$/   { print "File Trailer\n"; last; }
    when /.*/     { handle_batch_content($_); }
  }
}

sub handle_batch_content {
  my $batch_content = shift;
  print "Got $batch_content in $batchnum\n"; # or whatever else you want to do
}