You need to append to your array element instead of assigning to it anew in every loop iteration e.g

while($parser->get_tag){
    ## Note the . before = and the use of . instead of ,
    $stripped_html[0] .= $parser->get_trimmed_text() . "\n";
}
print $stripped_html[0];
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See. perlop for more info on the .= operator.