Not perfect, but its a good start

use strict;
use warnings;

@ARGV = 'GUTINDEX-2004.txt' unless @ARGV;

my $file = shift or die "Usage: $0 GUTINDEX-2001.txt";
my @data;

{
    open IN, '<', $file or die "Error: Can't open $file : $!";
    my $start_parsing;
    local $_;
    while(<IN>){
        next if /^\s*$/;
        last if /^End\s+of\s+\Q$file\E/;
        chomp;
    
        if( $start_parsing ){
            if(/^(\w+.*?) by (.+?)\s+(\d{5}.?)$/){
                my( $title, $author, $id ) = ( $1, $2, $3 );
                $data[$#data+1]->{id} = $id;
                $data[$#data]->{title} = $title;
                $data[$#data]->{author} = $author;
            } else {
                for( /\[(.+?)(\])?/g ){
                    my $foo = $1;
                    unless( $2 ){
                        while(<IN>){ 
                            if( /(.+?)\]/ ){
                                $foo .= $1;
                                last;
                            } else {
                                $foo .= <IN>
                            }
                        }
                    }
                    if( $foo =~ /\[?(\S+):(.+)\]?/s ){
                        $data[$#data]->{$1} = $2;
                    }
                    elsif( $foo =~ /,\s\d+$/s ){
                        $data[$#data]->{Date} = $foo;
                    }
                }
            }
        }
#        elsif( /^Title\s+and\s+Author/ ){
        elsif( /^\Q~ ~ ~ ~ Posting Dates for the below eBooks\E/ ){
            $start_parsing++;
        }
    }
}

use Data::Dumper;
print Dumper( \@data );
[download]

update:
What I noticed is that the first entry in GUTINDEX-2004.txt doesn't have a closing [ for [Subtitle:, which is a bug in the file.

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