Another suggestion to consider. Try not to use "but" because it is a flag that you are about to discount what you just said. Here's an alternative to the above, where you simply replace "but" with "and". (Also note saying why you like something really helps too.)

"Yes, I like that because it would help with X, and have you have you considered the cost?"

Usually "but" can be replaced with "and" fairly smoothly. However, if not, you can do something like:

"Yes, I like that because it would help with X. I wonder how much it would cost, both for the licenses and for the effort?"

oops, listen to F_J_H! The improv phrase is "yes, and" not "yes, but".

Funny you mention "Say yes". So far I often heard the advice to say "No" more often, especially during my apprenticeship.

When people advise you to say 'No' more often, I really think they mean to qualify your "Yes" so that you can actually follow through. For example, instead of saying "No" to your boss about the Perl, or even saying "No, because I don't know Perl", say a qualified "Yes." There are plenty of examples above of a qualified "Yes" that let you convey your doubts about the project without seeming uncooperative.

You can even get creative and say "Yes" in such a way that demonstrates you're not serious about the commitment. I used to do landscaping work in college, and when my boss discovered I was good with numbers, I took over part of the pricing for new jobs. When someone asked for a quote for something we could do but didn't want to (e.g. building a large retaining wall, mowing a 50 acre field in an inconvenient location), I'd give a "go away" price (usually by tripling our labor rate and padding the estimate) that would signal we didn't really want to take the job but didn't hurt our reputation in the same way a blanket "we don't do that" would.

This let the owner call back and say "Yes, we'd love to mow your field. We've run some numbers, and based on the location and the size of the job, it's going to cost you $2500. Oh, you'll pass? So sorry to hear that, have a nice day." Most of the time, people passed. The couple of times that someone took our "go away" price were rare enough that we were able to keep our focus on residential landscaping (which has much lower overhead) but frequent enough that we could still advertise we did larger scale jobs.

A qualified "Yes" will leave doors open when a "No" would slam them shut, and it's good to know how to say one in any field.