use CGI qw(:standard);    # easiest and safest way to do this
# ...
if (param()) {
    # Parameters were passed in: like button clicks,
    # text box contents, radio selections, etc.
    # Do something here like
    #    if (param('S1') eq 'Send') { ... }
} else {
    # Write HTML to produce the "baseline page"
    # Like
    # print "<input ...>"
}