use strict; 
use warnings; 
use Getopt::Long;

my $lfile;
my $bfile;

GetOptions (
    'l=s' => \$lfile,
    'b=s' => \$bfile
) or die;

die "Unexpected args: @ARGV" if @ARGV;
die "Must specify -l or -b" unless $lfile or $bfile;