Here's the code to reproduce it. Easier to see on windows, since it produces a buffer overflow. On linux, you would have to hook up a serial analyzer to see that the bytes don't get written (or are garbled). You have to put a valid serial in for $port (COM1, COM2, etc, on windows.../dev/ttyXXX on linux) to see the bug. There's a line to uncomment that will "cure" the bug with what should be essentially a no-op. Also, if you replace the $url with a non-https url, the bug doesn't show itself.

#!/usr/bin/perl
use strict;
use warnings;
#
# You must change what's below to a valid comm port, or the bug won't 
# show itself
#
my $port="COM9";
my $url='https://github.com';
use LWP::UserAgent;
my $serial;
BEGIN {
    if ($^O eq "MSWin32" or $^O eq "cygwin") {
        eval "use Win32::SerialPort";
        die "$@\n" if ($@); 
    } else {
        eval "use Device::SerialPort";
        die "$@\n" if ($@); 
    }
}
if ($^O eq "MSWin32" or $^O eq "cygwin") {
    $serial = Win32::SerialPort->new($port,1);
} else { 
    $serial = Device::SerialPort->new($port,1);
}
my $useragent = LWP::UserAgent->new();
my $request = new HTTP::Request;
$request->method("GET");
$request->url($url);
my $response = $useragent->request($request);
my $data = $response->decoded_content;
#
# the line below grabs the first 6 bytes, which would be <!DOCT
# but, the bug is triggered
#
$data=~s/(......).*/$1/s;
#
# now, uncomment the line below, and the bug doesn't show itself
# $data=substr($data,0);
#
print "data is [$data]\n";
$serial->baudrate(9600);
$serial->parity('none');
$serial->databits(8);
$serial->stopbits(1);
$serial->handshake('none');
$serial->write_settings();
$serial->write($data);
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