6.6. Matching Within Multiple Lines
6.6.1. Problem
You want to use regular expressions on a string containing more than one logical line, but the special characters . (any character but newline), ^ (start of string), and $ (end of string) don‘t seem to work for you. This might happen if you‘re reading in multiline records or the whole file at once.

6.6.2. Solution
Use /m, /s, or both as pattern modifiers. /s allows . to match a newline (normally it doesn‘t). If the target string has more than one line in it, /foo.*bar/s could match a "foo" on one line and a "bar" on a following line. This doesn‘t affect dots in character classes like [#%.], since they are literal periods anyway.

The /m modifier allows ^ and $ to match immediately before and after an embedded newline, respectively. /^=head[1-7]/m would match that pattern not just at the beginning of the record, but anywhere right after a newline as well.

perl学习之：匹配修饰符/s /m