sub search {
  my ($fh, $cmp) = @_;
  my ($lo, $hi) = (0, -s $fh);

  while (1) {
    my $mid = int(($lo + $hi)/2);
    if ($mid) {
      seek $fh, $mid-1, SEEK_SET;
      my $junk = <$fh>;
    } else { seek $fh , $mid, SEEK_SET; }
    my $start = tell $fh;
    my $rec = <$fh>;
    return unless defined $rec;
    chomp($rec);

    if ($hi == $lo) { 
      seek $fh, $start, SEEK_SET;
      return $rec 
    };
    local $_ = $rec;
    if ($cmp->($rec) < 0) { $lo = $mid+1 }
    else                  { $hi = $mid   }
  }
}

##</code><code>##

  open DICT, "<", "/usr/dict/Web2" or die...;
  if (search(\*DICT, sub { lc $_ cmp "foo" })) {
    # DICT is now positioned at the first word beginning with 'foo'
    while (<DICT>) {
      last unless /^foo/i;
      print;
    }
  }