What does Perl's modifier /m = multiline mean?

what does Perl's /m modifier means from this example?

For example, let say I have the following information in the Example.txt text file. And each line ends with the newline character with a 开发者_运维知识库data record of Data The input record separator is set to:

 $/="__Data__";

 Example.txt

 __Data__
 This is test A.\n
 This is test B.\n
 This is test C.\n
 This is test D.\n

Question 1, after changing the input record separator to Data, would the ^ and $ characters be position as follow?

  ^__Data__
  This is test A.\n
  This is test B.\n
  This is test C.\n
  This is test D.\n$

Question 2, let say I use the /m modifier while having the input record separator still set to Data, would the ^ and $ characters be set to the following?

  ^__Data__$
  ^This is test A.\n$
  ^This is test B.\n$
  ^This is test C.\n$
  ^This is test D.\n$

  if(/__Data__/m)
  {
      print;
  }

/$/ is not affected by $/.

Without /m,

• /^/ matches the starts of the string. (/(?-m:^)/ ⇔ /\A/)
• /$/ matches at the end of the string, and before a newline at the end of the string. (/(?-m:$)/ ⇔ /\Z/ ⇔ /(?=\n\z)|\z/)

^__Data__\n          "^" denotes where /(?-m:$)/ can match
This is test A.\n    "$" denotes where /(?-m:$)/ can match
This is test B.\n
This is test C.\n
This is test D.$\n$

With /m,

• /^/ matches the starts of the string and after a "\n". (/(?m:^)/ ⇔ /\A|(?<=\n)/)
• /$/ matches before a newline and at the end of the string. (/(?m:$)/ ⇔ /(?=\n)|\z/)

^__Data__$\n           "^" denotes where /(?m:^)/ can match
^This is test A.$\n    "$" denotes where /(?m:$)/ can match
^This is test B.$\n
^This is test C.$\n
^This is test D.$\n$

I was asked about

...$\n$

First, let's demonstrate:

>perl -E"say qq{abc\n} =~ /abc$/ ? 1 : 0"
1

>perl -E"say qq{abc\n} =~ /abc\n$/ ? 1 : 0"
1

The point is to allow /^abc$/ to match both "abc\n" and "abc".

>perl -E"say qq{abc\n} =~ /^abc$/ ? 1 : 0"
1

>perl -E"say qq{abc} =~ /^abc$/ ? 1 : 0"
1

Your assumption is correct, multiline causes ^ and $ to match the beginning and end of the string, whereas without it you match between newlines (and string ends).