From perldoc perlvar:

Each filehandle in Perl counts the number of lines that have been read from it.

Before you have read from the handle, $. is undefined. After the first read, it is 1.

---
print map { my ($m)=1<<hex($_)&11?' ':'';
$m.=substr('AHJPacehklnorstu',hex($_),1) }
split //,'2fde0abe76c36c914586c';
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