You shouldn't be surprised that something that reads from STDIN behaves differently when you redirect STDIN than when you don't.

"<" means "pretend what's in the file is being typed". When you don't specify a file to execute, perl reads the program from the STDIN. It stops reading when it receives and EOF signal or when it sees __END__ or __DATA__.

If your program proceeds to read from STDIN, so be it. What it reads has nothing to do with whether the program has used __DATA__ or not.

To show it's got nothing to do with __DATA__: 

BEGIN { print "Enter a line:\n";
        print "You entered: ", scalar(<>) }
print("a\n");
print("b\n");

[download]
>perl script.pl
Enter a line:
foo
You entered: foo
a
b

>perl < script.pl
Enter a line:
You entered: print("a\n");
b

[download]

In reply to Re^4: -p option with __DATA__ by ikegami
in thread -p option with __DATA__ by jdalbec

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