Help Understanding Script?

[samthemist]

I want to understand how exactly this script works. Basically, it returns every combination that is possible using '.' - for example:

sam
s.am
sa.m
s.a.m

Can you explain what exactly it is going through on each step? I understand some of it, but not others - like $combinations = 1 << ($strlength - 1);

sub isBitSet { if ($_[0] & (1 << ($_[1] - 1))) { return 1; } else { return 0; }}
 
sub possibilize {
  $strlength = length($_[0]);
  if ($strlength > 20) { return 0; }
  $combinations = 1 << ($strlength - 1);
  $strloc = 0; $str = "";
  for ($x = 0; $x < $combinations; $x++) {
    $strloc = 1; $str = $_[0];
    for ($y = 1; $y < $strlength; $y++) {
      $str = substr($str, 0, $strloc);
      if (isBitSet($x, $y)) { $str.= "."; $strloc++; } 
      $str.= substr($_[0], $y); $strloc++;
    } $out.= $str.
  } return $out;
}

[samthemist]

Can anyone explain?
...