Sounds like you need to apply some form of pattern recognition. A crude method could (presuming that what you meant to say was that at minimum some number of your desired characters must exist in the target),

• Start with any six letter characters
• Identify if ANY of your target letters are in the six, if not, move to the next six characters
• If so, determine if that match meets your minimum requirements for a successful match, store and report the find

Fortunately (or unfortunately depending upon your perspective), pattern recognition in strings is quite an involved subject. You might consider shopping around for some algorithms to either implement or search CPAN for.