Here's a solution that does it all in perl:

#!/usr/bin/perl

use warnings;
use strict;
use File::Find;
use Tie::File;

find (\&replace,"/path/to/directory");

sub replace {
    my $cur_file=$File::Find::name;
    return if (!-f $cur_file || !-w $cur_file);

    tie my @file_content,'Tie::File',$cur_file
        or die "Couldn't tie file $cur_file";

    for (@file_content) {
        s/this phrase/that locution/g;
    }
}
[download]

You may want to read the Tie::File perldoc regarding file locking and memory consumption.

And just for completeness, here's a solution that does it all without perl:

find this/ -type f -exec sed -i 's/gg/dd/g' {} \;
[download]