sub getLinkParse
   {  use LWP::UserAgent; use HTTP::Request::Common;
      $sentence = "This is a test sentence.";
      $response = (new LWP::UserAgent)->request(POST
            'http://www.link.cs.cmu.edu/cgi-bin/link1/construct-page-4.cgi#submit',
            [ Sentence      => $sentence,
              Constituents  => "1", <<<< this is a checkbox
              NullLinks     => "1", <<<< this is a checkbox
              AllLinkages   => "OFF",
              LinkDisplay   => "on",
              ShortLength   => "6",
              PageFile      => "/docs/submit-sentence-4.html",
              InputFile     => "/scripts/input-to-parser",
              Maintainer    => "sleator\@cs.cmu.edu" ] );

      exit -1 unless $response->is_success;
      $_ = $response->{_content};
      print $_;
}

This is the origianl form on the site:

<FORM METHOD="POST" ACTION="/cgi-bin/link1/construct-page-4.cgi#submit">
<textarea name="Sentence" MAXLENGTH=200  wrap=virtual rows=3 cols=70></textarea><br>
<INPUT TYPE="checkbox" NAME="Constituents" CHECKED>Show constituent tree &nbsp;
<INPUT TYPE="checkbox" NAME="NullLinks" CHECKED>Allow null links &nbsp;
<INPUT TYPE="checkbox" NAME="AllLinkages" OFF>Show all linkages &nbsp;
<INPUT TYPE="HIDDEN" NAME="LinkDisplay" VALUE="on">
<INPUT TYPE="HIDDEN" NAME="ShortLength" VALUE="6">
<INPUT TYPE="HIDDEN" NAME="PageFile" VALUE="/docs/submit-sentence-4.html">
<INPUT TYPE="HIDDEN" NAME="InputFile" VALUE="/scripts/input-to-parser">
<INPUT TYPE="HIDDEN" NAME="Maintainer" VALUE="sleator@cs.cmu.edu">
<br>
<INPUT TYPE="submit" VALUE="Submit one sentence">
<br>
</FORM>