For instance, is "<>" short hand for "<STDIN>"?.

<> is a very common and useful shorthand notation. You'll often encounter this, and I recommend getting familiar with it, it really comes in handy every time you want to process user-supplied input.

The short version is that it Does What You Mean; I/O Operators has the nitty-gritty:

The null filehandle <> is special: it can be used to emulate the behavior of sed and awk, and any other Unix filter program that takes a list of filenames, doing the same to each line of input from all of them. Input from <> comes either from standard input, or from each file listed on the command line. Here's how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is empty, $ARGV[0] is set to "-", which when opened gives you standard input. The @ARGV array is then processed as a list of filenames. The loop

while (<>) {
    ...            # code for each line
}
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is equivalent to the following Perl-like pseudo code:

unshift(@ARGV, '-') unless @ARGV;
while ($ARGV = shift) {
    open(ARGV, $ARGV);
    while (<ARGV>) {
        ...        # code for each line
    }
}
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except that it isn't so cumbersome to say, and will actually work. [...]

For the various flags to the perl executable, see perlrun. -n and -p put certain loops around your code, allowing you to write only the code inside the loop (very useful for one-liners on the command line); -l auto-chomps your input, which is useful in much the same situations.