Perl: Match multiple lines of text

Generally, regular expressions are for single-line text. Since Perl can process strings of any length, the Perl mode can easily match multiple lines of text, just like a single line of text. Of course, the expression should contain multiple lines of text. The following strings contain four rows:
$ _ = "I'm much better \ nthan Barney is \ Nat bowling, \ nwilma, \ n ";
Anchor ^ and $ indicate the start and end of the entire string, enhancing understanding. , ^And $ refer to the beginning and end of a string, not for a single row.However, the regular expression option/M allows them to match based on internal linefeeds (/M indicates multiple rows ). At this time, the anchor is targeted at each row, not the entire string. Therefore, this pattern can match:
Print "found 'wilm' at start of line \ n" if/^ Wilma \ B/IM;
Similarly, you can replace a single row in a multi-line string. In the following example, we read the entire file into a variable and add the file name to the beginning of each line:
Open File, $ filename
Or die "can't open '$ filename': $ !";
My $ lines = join '', <File>;
$ Lines = ~ S/^/$ filename:/GM;