split with no argument in perl

I'm new to perl, and I wonder what this line of code mean?

($q,$dummy, $d,$v) = split;

I search through google, but i found no explanation of using split without argument, does this kind of use related to the "while" block?

And the full code fragment is:

open(T,"$opt_judgments") ||  die "can't open judgment file: $opt_judgments\n";
while (<T>) {
  if ($opt_trec) {
    ($q,$dummy, $d,$v) = split;
  } else {
    ($q,$d,$v) = split;
  }
  $dict{$q ."=".$d} =$v;
  if ($v != 0) {
    $totalRels{$q开发者_高级运维} ++;
  }
}

It splits the current line ($_) on whitespace. Quoting the manual:

If EXPR is omitted, splits the $_ string. If PATTERN is also omitted, splits on whitespace (after skipping any leading whitespace).

From perldoc:

The general syntax of split is:

split /PATTERN/,EXPR

If EXPR is omitted, it splits the $_ string.

If PATTERN is also omitted, splits on whitespace (after skipping any leading whitespace). Anything matching PATTERN is taken to be a delimiter separating the fields. (Note that the delimiter may be longer than one character.)

Since in your case both PATTERN and EXPR are omitted. A split of $_ on whitespace occurs and the first four pieces of the split are assigned to $q, $dummy, $d and $v respectively.