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>;
