use HTML::Entities qw/encode_entities/;
my $SOAPResult = q{ "Hello" <world> &amp; };
encode_entities($SOAPResult);
print <<"EndOfText";
  <input type="text" name="mytext" id="mytext" value="$SOAPResult"/>
EndOfText

__END__

  <input type="text" name="mytext" id="mytext" value=" &quot;Hello&quot; &lt;world&gt; &amp;amp; "/>