Had some horror to migrate my old Thunderbird archive to IMAP.. found this script. I have made some additions to this code:

- replace '.' (dots) by '_' in folder names

- subscribe to the folders !

- create (final) dir on other location

- appended a base target folder

use warnings;
use strict;
use Data::Dumper;
use Mail::MboxParser;
use Mail::IMAPClient;

my $base_dir    = ".thunderbird/profile_path/Mail/Local Folders/";
my $base_target = "INBOX";
my $imap_server = "localhost";
my $imap_user   = "username";
my $imap_pass   = "password";

my $parseropts = {
        enable_cache    => 0,
        enable_grep     => 0,
        cache_file_name => 'cache-file',
};

my $skip_deleted = 1;

my $imap = Mail::IMAPClient->new(
                        Server   => $imap_server,
                        User     => $imap_user,
                        Password => $imap_pass,
)       or die "Cannot connect to $imap_server as $imap_user: $@";

parse_dir($base_dir);

sub parse_dir {
    my $dir = shift;
    opendir my $dir_h, $dir
                    or die "Unable to opendir $dir: $!\n";
    print "Reading directory $dir\n";


    ##Dirty, probably IMAP-server dependent stuff.
    #This stuff is for dovecot.
    my $temp_dir = $dir;
    $temp_dir =~ s!^$base_dir!$base_target!;
    $temp_dir =~ s!\.sbd!!g;
    $temp_dir =~ s!/+!/!g;
    $temp_dir =~ s!\.+!_!g;
    $temp_dir =~ s!^/!!;
    $temp_dir =~ s!/ *!.!g;
    if ($temp_dir ne $base_target) {
        print "================================= Making dir $temp_dir\
+n";
        $imap->create($temp_dir)
                        or warn "(A) unable to create $temp_dir: $@\n"
+;
        $imap->subscribe($temp_dir)
                        or warn "(A) subscribe to $temp_dir: $@\n";
        print "\n";
    }
    
    foreach my $directory (grep /\.sbd$/, readdir $dir_h) {
        parse_dir($dir."/".$directory);
    }
    seekdir($dir_h, 0);
    foreach my $mail_file (grep !/^\./, grep !/(\.html|\.sbd|\.msf|\.d
+at)$/, readdir $dir_h) {
        my $mf = $dir."/".$mail_file;
        print "Going to parse $mf\n";
        my $mb = Mail::MboxParser->new($mf,
                                                                      
+  decode     => 'ALL',
                                                                      
+  parseropts => $parseropts);
    
        my $folder_name = $temp_dir.".".$mail_file;
        $folder_name =~ s!/+!/!g;
        $folder_name =~ s!\.+!_!g;
        $folder_name =~ s!^/!!;
        $folder_name =~ s!/ *!.!g;
        print "================================= Making dir $folder_na
+me\n";
        $imap->create($folder_name)
                        or warn "(B) unable to create $folder_name: $@
+\n";
        $imap->subscribe($folder_name)
                        or warn "(B) subscribe to $folder_name: $@\n";
    
        for my $msg ($mb->get_messages) {
            #Skip deleted messages...
            unless($skip_deleted and (hex($msg->header->{"x-mozilla-st
+atus"}) & 0x0008)) {
                print "Appending msg " . $msg->header->{subject} . " t
+o $folder_name\n";
                unless($imap->append_string($folder_name, $msg, '\Seen
+')) {
                                warn "Couldn't append " . $msg->header
+->{subject} . " to $folder_name: $@\n";
                                warn "Skipping\n";
                                next;
                }
            } else {
                warn "Skipping " . $msg->header->{subject} . " - delet
+ed message\n";
            }
    
        }
    }
    closedir($dir_h);

}
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