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;