If use locale is in effect, the case map is taken from the current locale. See the perllocale manpage.

The /s and /m modifiers both override the $* setting. That is, no matter what $* contains, /s without /m will force ``^'' to match only at the beginning of the string and ``$'' to match only at the end (or just before a newline at the end) of the string. Together, as /ms, they let the ``.'' match any character whatsoever, while yet allowing ``^'' and ``$'' to match, respectively, just after and just before newlines within the string.

These are usually written as ``the /x modifier'', even though the delimiter in question might not actually be a slash. In fact, any of these modifiers may also be embedded within the regular expression itself using the new (?...) construct. See below.

The /x modifier itself needs a little more explanation. It tells the regular expression parser to ignore whitespace that is neither backslashed nor within a character class. You can use this to break up your regular expression into (slightly) more readable parts. The # character is also treated as a metacharacter introducing a comment, just as in ordinary Perl code. This also means that if you want real whitespace or # characters in the pattern (outside of a character class, where they are unaffected by /x), that you'll either have to escape them or encode them using octal or hex escapes. Taken together, these features go a long way towards making Perl's regular expressions more readable. Note that you have to be careful not to include the pattern delimiter in the comment--perl has no way of knowing you did not intend to close the pattern early. See the C-comment deletion code in the perlop manpage.

关于 /m/s 给出一个合理的解释：（通过现象分析实质）

By default, the ``^'' character is guaranteed to match at only the beginning of the string, the ``$'' character at only the end (or before the newline at the end) and Perl does certain optimizations with the assumption that the string contains only one line. Embedded newlines will not be matched by ``^'' or ``$''. You may, however, wish to treat a string as a multi-line buffer, such that the ``^'' will match after any newline within the string, and ``$'' will match before any newline. At the cost of a little more overhead, you can do this by using the /m modifier on the pattern match operator. (Older programs did this by setting $*, but this practice is now deprecated.)

To facilitate multi-line substitutions, the ``.'' character never matches a newline unless you use the /s modifier, which in effect tells Perl to pretend the string is a single line--even if it isn't. The /s modifier also overrides the setting of $*, in case you have some (badly behaved) older code that sets it in another module.

当有.出现在匹配换行符的位置的时候，  那么就将正则在解析的时候 /s 的优先级要高，也就是将字符串进行 sigle line  的解析了。
当出现 ^ 或者 $ 来匹配开始位置和结束位置的时候，即使这个时候也出现了 . 符号来匹配换行,正则在解析的时候 /m 的优先级要搞，也就是将字符串进行 multiple  lines 的解析了。

在 multiple lines 中 . 符号是永远也不会用来匹配 newline 的，也就是 /m 的优先级屏蔽了 . 符号对于 newline 的匹配，如果要使 . 能够匹配 newline, 那么请使用 /s