Have a look at the examples in Perl I/O Operators. I'd recommend you read the entire section, but the most relevant snippets that most directly answer your question are probably:

In scalar context, evaluating a filehandle in angle brackets yields the next line from that file (the newline, if any, included), or undef at end-of-file or on error.

...

The following lines are equivalent:

        while (defined($_ = <STDIN>)) { print; }
        while ($_ = <STDIN>) { print; }
        while (<STDIN>) { print; }
        for (;<STDIN>;) { print; }
        print while defined($_ = <STDIN>);
        print while ($_ = <STDIN>);
        print while <STDIN>;
[download]