How to use a checkbox

I want to use a checkbox in Perl to make some operations.

The code is like this:

print checkbox( -name=>'candidate', -value=>'ON' );

print param('candidates');

if ( param('candidates') eq "ON" ) {
    ...
}

I want to know what exactly is param('candidates')?

There is nothing displayed in the web browser, and the following if statement is not executed.

Can anyone give me some hint how to fix it?