As any good manager knows, you shouldn't micromanage your employees. Just tell them what you want, and let them figure out the best way of doing it. Similarly, it's often best to think of a regular expression as a kind of specification: "Here's what I want; go find a string that fits the bill."
On the other hand, the best managers also understand the job their employees are trying to do. The same is true of pattern matching in Perl. The more thoroughly you understand of how Perl goes about the task of matching any particular pattern, the more wisely you'll be able to make use of Perl's pattern matching capabilities.
One of the most important things to understand about Perl's pattern-matching is when not to use it.
When people of a certain temperament first learn regular expressions, they're often tempted to see everything as a problem in pattern matching. And while that may even be true in the larger sense, pattern matching is about more than just evaluating regular expressions. It's partly about looking for your car keys where you dropped them, not just under the streetlamp where you can see better. In real life, we all know that it's a lot more efficient to look in the right places than the wrong ones.
Similarly, you should use Perl's control flow to decide which patterns to execute, and which ones to skip. A regular expression is pretty smart, but it's smart like a horse. It can get distracted if it sees too much at once. So sometimes ...