#!/usr/bin/perl -w

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

use constant FILEHDR => 4;
use constant CDRLEN  => 286;

my ($trace, $help, $infile);
my $swap = '';
my $indir  = getcwd;
my $outdir = getcwd;

GetOptions (
    "h|help"    => \$help,
    "filename|f=s"    => \$infile,
    "swap|s"    => \$swap,
    "input|i=s"    => \$indir,
    "output|o=s"    => \$outdir,
    "trace|t"    => \$trace
) or usage();


sub usage {
    exit;
}

my $outfile = $infile;
my $data;

if ($infile) {
    #open (OUTPUT, ">$outdir/$outfile");
    open (DATA, "$indir/$infile");
    binmode DATA;
    
    until (eof DATA) {
        read (DATA, $data, 2);
        my $tag    = unpack "H2", substr $data,0,1,'';
        my $length = unpack "C", substr $data,0,1,'';
        $length -= 2;

        if ($length == "81") {
            $length = unpack "C", substr $data,0,1,'';
            $length -= 1;
        }

        if ($length > 0) {
            read (DATA, $data, $length);

            my $rec      = unpack "H*", substr $data,0,$length,'';

            printf ("RECORD TAG    : %s\n", $tag);
            printf ("RECORD LENGTH : %s\n", $length);
            printf ("RECORD        : %s\n", $rec);
        }

    }

    close(DATA);
    #close(OUTPUT);
}
[download]

Without actually testing your code, that seems to be OK. A couple of things to improve it though: you should always test the return value from open,

open (DATA, "$indir/$infile") or die "Unable to open $indir/$infile: $
+!";
[download]

would be a conventional way of doing that. Notice also that read returns the number of bytes read, which you can use to check that you get back the number you expect - if data can be wrong it will be!