Note that this will deal with Mary and her little lamb over any line boundaries:

c:\@Work\Perl\monks>perl
use strict;
use warnings;

use autodie;

my $data = <<EOD;
Mary Had
A Little Lamb She
Was GOOD
Not Changing this
Or This
Mary Had A Little
Lamb She Was GOOD
Line 9
Line 10
EOD

open my $fh, '<', \$data;

my $content = do { local $/;  <$fh>; };

print "[[$content]] \n\n";

$content =~
  s{ Mary \s+ Had \s+ A \s+ Little \s+ Lamb \s+ She \s+ Was \s+ GOOD }
   {CHANGED!!!}xmsg;

print "<<$content>> \n";

__END__
[[Mary Had
A Little Lamb She
Was GOOD
Not Changing this
Or This
Mary Had A Little
Lamb She Was GOOD
Line 9
Line 10
]]

<<CHANGED!!!
Not Changing this
Or This
CHANGED!!!
Line 9
Line 10
>>
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