Ok, we are almost there, you need again a module and a very light change. I tried not to change your code too much just to get it work on my machine.
You need to change, actually add one line to this part:

 
use IO::All;    # install this module first

push @parts, Email::MIME->create(
    attributes  => {
        filename        => $attachments{doc_name},
        content_type    => $attachments{doc_mime_type},
        disposition     => 'attachment',
        encoding        => 'base64',
        name            => $attachments{doc_name}
    },
        body => io($attachments{doc_name})->all,  # add this line
);
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For the illegal seek error, just delete these lines, you don't need them:

#print "error1 $!\n";
#print "error2 $Mail::Sendmail::error\n";
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Cheers, have fun!