Perl5 リファレンス

       readline EXPR
               Reads from the filehandle whose typeglob is
               contained in EXPR.  In scalar context, each call
               reads and returns the next line, until end-of-file
               is reached, whereupon the subsequent call returns
               undef.  In list context, reads until end-of-file
               is reached and returns a list of lines.  Note that
               the notion of "line" used here is however you may
               have defined it with `$/' or
               `$INPUT_RECORD_SEPARATOR').  See the section on
               "$/" in the perlvar manpage.

               When `$/' is set to `undef', when readline() is in
               scalar context (i.e. file slurp mode), and when an
               empty file is read, it returns `''' the first
               time, followed by `undef' subsequently.

               This is the internal function implementing the
               `' operator, but you can use it directly.
               The `' operator is discussed in more detail
               in the I/O Operators entry in the perlop manpage.

                   $line = ;
                   $line = readline(*STDIN);           # same thing