Yes, while (<STDIN>) { ... } is special in that it:

1. Reads a line from STDIN
2. Assigns the line to $_
3. Exits the loop if STDIN was at EOF before the read

Invoking <STDIN> by itself just reads a line but doesn't automatically assign it to $_.

Note that this is different from merely testing whether or not $_ is true as is shown by this example:

$/ = "0";
while (<DATA>) {
  print "got $_ and it ", ($_ ? "is" : "is not"), " true\n";
}
__END__
00000
[download]

which emits:

got 0 and it is not true
got 0 and it is not true
...
[download]