To elaborate a bit what should probably be happening is that the regex engine should always backtrack on .* into the equivalent of NOTHING, but instead it commits to the match and fails under /m once it reaches \n. 
perl5.9.5 -Mre=debug -e 'my @re = (qr/.*\z/, qr/.?\z/, qr/(|.+)\z/); "\n" ~~ $_ for @re'

Compiling REx ".*\z"
Final program:
   1: STAR (3)
   2:   REG_ANY (0)
   3: EOS (4)
   4: END (0)
floating ""$ at 0..2147483647 (checking floating) anchored(MBOL) implicit minlen 0 
Compiling REx ".?\z"
Final program:
   1: CURLY {0,1} (4)
   3:   REG_ANY (0)
   4: EOS (5)
   5: END (0)
floating ""$ at 0..1 (checking floating) minlen 0 
Compiling REx "(|.+)\z"
Final program:
   1: OPEN1 (3)
   3:   BRANCH (5)
   4:     NOTHING (8)
   5:   BRANCH (FAIL)
   6:     PLUS (8)
   7:       REG_ANY (0)
   8: CLOSE1 (10)
  10: EOS (11)
  11: END (0)
floating ""$ at 0..2147483647 (checking floating) minlen 0 
Guessing start of match in sv for REx ".*\z" against "%n"
Found floating substr ""$ at offset 0...
Position at offset 0 does not contradict /^/m...
Guessed: match at offset 0
Matching REx ".*\z" against "%n"
   0 <> <%n>                 |  1:STAR(3)
                                  REG_ANY can match 0 times out of 2147483647...
   0 <> <%n>                 |  3:  EOS(4)
                                    failed...
                                  failed...
Match failed
Guessing start of match in sv for REx ".?\z" against "%n"
Found floating substr ""$ at offset 0...
Guessed: match at offset 0
Matching REx ".?\z" against "%n"
   0 <> <%n>                 |  1:CURLY {0,1}(4)
                                  REG_ANY can match 0 times out of 1...
   0 <> <%n>                 |  4:  EOS(5)
                                    failed...
                                  failed...
   1 <%n> <>                 |  1:CURLY {0,1}(4)
                                  REG_ANY can match 0 times out of 1...
   1 <%n> <>                 |  4:  EOS(5)
   1 <%n> <>                 |  5:  END(0)
Match successful!
Guessing start of match in sv for REx "(|.+)\z" against "%n"
Found floating substr ""$ at offset 0...
Guessed: match at offset 0
Matching REx "(|.+)\z" against "%n"
   0 <> <%n>                 |  1:OPEN1(3)
   0 <> <%n>                 |  3:BRANCH(5)
   0 <> <%n>                 |  4:  NOTHING(8)
   0 <> <%n>                 |  8:  CLOSE1(10)
   0 <> <%n>                 | 10:  EOS(11)
                                    failed...
   0 <> <%n>                 |  5:BRANCH(8)
   0 <> <%n>                 |  6:PLUS(8)
                                  REG_ANY can match 0 times out of 2147483647...
                                  failed...
   1 <%n> <>                 |  1:OPEN1(3)
   1 <%n> <>                 |  3:BRANCH(5)
   1 <%n> <>                 |  4:  NOTHING(8)
   1 <%n> <>                 |  8:  CLOSE1(10)
   1 <%n> <>                 | 10:  EOS(11)
   1 <%n> <>                 | 11:  END(0)
Match successful!
Freeing REx: ".*\z"
Freeing REx: ".?\z"
Freeing REx: "(|.+)\z"

In reply to Re: Strange regex to test for newlines: /.*\z/ by avar
in thread Strange regex to test for newlines: /.*\z/ by betterworld

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