Without knowing too much about Word, I can only recommend to Use The Macro Recorder, Luke (Re: Using Win32::OLE and Excel - Tips and Tricks). If you can record a Word macro doing what you do, you can write a Perl program to do the same.

use strict;
use warnings;
use Win32::OLE::Const 'Microsoft Word';

$Win32::OLE::Warn = 1;  

my $word = Win32::OLE->new("Word.Application");

$word->{DisplayAlerts} = 0;
$word->{Visible} = 0;

my $doc = $word->Documents->Open("\\YOUR\\PATH\\TO\\FILE");
my $selection = $doc->Content->Find;

$selection->{Text} = "hippopotamus";
$selection->{Replacement}->{Text} = "elephant";
$selection->{MatchCase} = 1;
$selection->Execute({Replace => wdReplaceAll});

$doc->SaveAs("\\YOUR\\PATH\\TO\\ALTEREDFILE");
$doc->Close;

$word->Quit;
[download]

Thanks a lot! Creating documents (xls, doc) is possible, but I was not sure if it was possible to manipulate excisting documents. Performance issues with a lot of documents? I'm very glad with the code (and it ain't VB ;-) ....)

Performance issues with a lot of documents?

Shouldn't be. But the specific answer depends on your performance requirements and your definition of "a lot". (358 files doesn't seem like a lot to me.) Just test and see.

...but I was not sure if it was possible to manipulate excisting documents.

Corion's advice about macros is dead on. And after you have seen some of the macro code side by side with the equivalent Perl code, it gets easier to translate between the two. The Word Object Model Reference is a handy information source.