In fact Eily is precise as always elevenfly, and he pointed exactly where $. is special in oneliners.

He gave you the definition, i'll add an example. Given a.txt and b.txt as following:

#cat a.txt
a file line 1
a file line 2
a file line 3

#cat b.txt
b file line 1
b file line 2
b file line 3
[download]

if you use $. to check line number (and $. is implicit for a bare .. flip-flop operator), you have:

#perl -ne "print if 1..2" a.txt b.txt
a file line 1
a file line 2
[download]

and so is because $. does not reset automatically for an implicit close of the filehandle (and -n iterates across file given as arguments and reopen each time ARGV so without explicitly closing it).

But if you use the right idiom close ARGV if eof everything runs as expected: $. is reset by the explicit close that happens only if eof is encounterd:

#perl -ne "print if 1..2; close ARGV if eof" a.txt b.txt
a file line 1
a file line 2
b file line 1
b file line 2
[download]

L*

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