FLV::Info で ustream の FLV ファイル情報をとろうとするとエラーになる件

#!/usr/bin/perl

use strict;
use warnings;
use Readonly;

Readonly my $FLV_TAG_TYPE_AUDIO                => 0x08;
Readonly my $AUDIO_MASK_STEREO                 => 0x01;
Readonly my $AUDIO_MASK_16bit                  => 0x02;
Readonly my $AUDIO_MASK_RATE                   => 0x0c;
Readonly my $AUDIO_RATE_5point5KHZ             => 0x00;
Readonly my $AUDIO_RATE_11KHZ                  => 0x04;
Readonly my $AUDIO_RATE_22KHZ                  => 0x08;
Readonly my $AUDIO_RATE_44KHZ                  => 0x0c;
Readonly my $AUDIO_MASK_FORMAT                 => 0xf0;
Readonly my $AUDIO_FORMAT_UNCOMPRESSED         => 0x00;
Readonly my $AUDIO_FORMAT_ADPCM                => 0x10;
Readonly my $AUDIO_FORMAT_MP3                  => 0x20;
Readonly my $AUDIO_FORMAT_NELLYMOSER_8KHZ_MONO => 0x50;
Readonly my $AUDIO_FORMAT_NELLYMOSER           => 0x60;

Readonly my %AUDIO_RATE => (
    $AUDIO_RATE_5point5KHZ => 5.5,
    $AUDIO_RATE_11KHZ      => 11,
    $AUDIO_RATE_22KHZ      => 22,
    $AUDIO_RATE_44KHZ      => 44,
);

Readonly my %AUDIO_FORMAT => (
    $AUDIO_FORMAT_UNCOMPRESSED         => 'Uncompressed',
    $AUDIO_FORMAT_ADPCM                => 'ADPCM',
    $AUDIO_FORMAT_MP3                  => 'MP3',
    $AUDIO_FORMAT_NELLYMOSER_8KHZ_MONO => 'Nellymoser 8kHz mono',
    $AUDIO_FORMAT_NELLYMOSER           => 'Nellymoser',
);

my $file = shift;

open my $fh, '<', $file;

my $header = read_header($fh);

while ( ! eof $fh ) {
    my $tag = read_tag($fh);
    if ( $tag->{type} == $FLV_TAG_TYPE_AUDIO and $tag->{length} ) {
        my $first_byte = $tag->{first_byte};


  my $stereo       = $first_byte & $AUDIO_MASK_STEREO;
  my $audio_size  = $first_byte & $AUDIO_MASK_16bit ? 16 : 8;
        my $audio_rate   = $AUDIO_RATE{ $first_byte & $AUDIO_MASK_RATE } || 'unknown';
        my $audio_format = $AUDIO_FORMAT{ $first_byte & $AUDIO_MASK_FORMAT } || 'unknown';

        print 'audio type: ' . ( $stereo ? 'stereo' : 'mono' ) . "\n";
        print "audio size: $audio_size bit\n";
        print "audio rate: $audio_rate Hz\n";
        print "audio format: $audio_format\n";
        last;
    }
}

sub read_header {
    my $fh = shift;
    read $fh, my $sig,       3;
    read $fh, my $version,   1;
    read $fh, my $flag,      1;
    read $fh, my $offset,    4;
    read $fh, my $prev_tag,  4;

    return {
        sig => $sig,
        version => unpack('H*', $version),
        flag    => unpack('H*', $flag),
        offset  => unpack('H*', $offset),
    };
}

sub read_tag {
    my $fh = shift;
    read $fh, my $type,               1;
    read $fh, my $length,             3;
    read $fh, my $timestamp,          3;
    read $fh, my $timestamp_extended, 1;
    read $fh, my $stream_id,          3;
    read $fh, my $first_byte,         1;

    my $pos = hex unpack 'H*', $length;
    seek $fh, $pos - 1, 1;

    read $fh, my $prev_tag,  4;

    return {
        type       => unpack('C', $type),
        length     => $pos,
        first_byte => unpack('C', $first_byte),
    };
}