#!/usr/bin/perl
use strict;
use warnings;
use IO::All -utf8; # Turn on utf8 for all io

my $contents = io->file('in.txt')->slurp;
$contents =~ s/The action failed\./failed_build/g;
$contents > io('in.txt');

__END__

$ cat in.txt
one
two
The action failed.
three
The action failed.  twice The action failed.
four

$ cat in.txt
one
two
failed_build
three
failed_build  twice failed_build
four