The current input line number for the last file handle from which you read (or performed a
tell on). An explicit close on a filehandle resets the line number. Because "
<>" never does an explicit close, line numbers increase across ARGV files (but see examples under eof()). Localizing
$. has the effect of also localizing Perl's notion of "the last read filehandle". (Mnemonic: many programs use "." to mean the current line number.)