Find & Replace within the files of a folder tree recursively

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Does someone have a tool to replace one string by some other string 
within all files of a folder tree recursively?
(I once wrote a perl script for this, but it only functioned if I copied 
each file to some other destination.)

Now I just RTFM and found these beautiful scripts in the Perl Cookbook, 
p. 323:
##################################################
##
##   Type alle the names and paths of the files within a folder tree:
##
##################################################
use File::Find;
@ARGV = qw(.) unless @ARGV;
find sub {  print $File::Find::name, -d && '/', "\n" }, @ARGV;
#################### E N D ########################

and

##################################################
##
##   Find the biggest file within a folder tree:
##
##################################################
use File::Find;
@ARGV = qw(.) unless @ARGV;
my ($saved_size, $saved_name) = (-1, '');

sub biggest  {
  return unless -f && -s _ > $saved_size;
  $saved_size = -s _;
  $saved_name = $File::Find::name;
  }
find (\&biggest, @ARGV);
print "Biggest file $saved_name in @ARGV has $saved_size bytes length.\n";
#################### E N D ########################


Now again my question:
Can someone add some lines, so I can do search-and-replace within each file?

TIA,
Detlef Lindenthal




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