You need some javascript to do this. In your header include the following:
<script language='text/javascript'>
var oText=document.getElementById('MyTextField');
function toggle_display(n)
{
if (n=='show') {
oText.style.visibility='visible';
} else
if (n=='hide') {
oText.style.visibility='hidden';
}
}
</script>
And in your CGI.pm generated text fields, include properties
for the controlling field:
onchange=>'toggle_display(this.value)'
for MyTextField:
id=>'MyTextField'
It's really more of a javascript/css problem, but it can be solved within the CGI.pm framework.
HTH,
SSF