To further elaborate on hv's excellent response, I often want to know for a fact that a value is explicitly TRUE or explicitly FALSE. Some situations:

• Talking with another program through an interface
• Passing values through an API, like HTML::Template
• Dealing with people who are cute and like to use the zero-but-true value "0E0"
• Making it explicit to the maintainer (who is usually me, 6 months down the road) exactly what this is meant for
• Doing validation checks so that I can be certain something is useful only in a boolean context.

It's a minor point, but it's those details that I find separate the beginner from the expert.