Because the filepointer is in sync with the parsing phases of Perl

Not necessarily, no... Let's see

BEGIN {
    print "BEGIN\n";
}
UNITCHECK {
    print "UNITCHECK\n";
}
CHECK {
    print "CHECK\n";
}
INIT {
    print "INIT\n";
}
print "MAIN\n";
[download]

Let's strace it (Debian Jessie, Perl 5.22)

open("t.pl", O_RDONLY)                  = 3
...
read(3, "BEGIN {\n    print \"BEGIN\\n\";\n}\nU"..., 8192) = 146
write(1, "BEGIN\n", 6)                  = 6
read(3, "", 8192)                       = 0
close(3)                                = 0
write(1, "UNITCHECK\n", 10)             = 10
write(1, "CHECK\n", 6)                  = 6
write(1, "INIT\n", 5)                   = 5
write(1, "MAIN\n", 5)                   = 5
[download]

Note that the file is closed right after BEGIN (and before UNITCHECK). So no handle to it can exist. But, if we add

__DATA__
whatever
[download]

that it changes a bit...

open("t.pl", O_RDONLY)                  = 3
...
read(3, "BEGIN {\n    print \"BEGIN\\n\";\n}\nU"..., 8192) = 165
write(1, "BEGIN\n", 6)                  = 6
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
write(1, "UNITCHECK\n", 10)             = 10
write(1, "CHECK\n", 6)                  = 6
write(1, "INIT\n", 5)                   = 5
write(1, "MAIN\n", 5)                   = 5
...
close(3)                                = 0
exit_group(0)                           = ?
[download]

Now the file isn't closed until exit. So without __DATA__ there is no handle to the file, and the only handle (that I can find :) is DATA.

Anyway, let us know if you'll find something in perl5db.pl or somewhere else.

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