# a unix/linux version:
Traverse('./');
sub Traverse{
    my $dir = shift;
    opendir my $dh, $dir or die "$!, for $dir";
    for my $file ( grep !/^\./, readdir $dir ) { # but adjust for OS
        my $path = "$dir/$file";
        if ( -d $path ) {
            Traverse( $path );
        }
        else {
            ( $file =~ /\.txt$/ ) and Process( $path );
            # or for VMS /\.TXT\;\d+$/
        }
    }
    closedir $dh;
}
sub Process{
    my $file = shift;
    open my $fh "<$file" or die "$!, for $file";
    my @updated = ();
    while( <$fh> ) {
        s/find/replace/g;
        push @updated;
    }
    close $fh;
    open $fh, ">$file" or die "$!, for $file";
    print $fh @updated;
    close $fh;
}