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 $INPUT_RECORD_SEPARATOR). See "$/" in perlvar.
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 <EXPR> operator, but you can use it directly. The <EXPR> operator is discussed in more detail in "I/O Operators" in perlop.
$line = <STDIN>;
$line = readline(*STDIN); # same thing