#! perl
use strict;
use warnings;
use Data::Dump;
use YAML::XS;

my $hash = { Fred => 'Wilma', Frederick => 'Mary', Barney => 'Fred' };
my $yaml =   Dump $hash;
   $yaml =~  s/\bFred\b/FRED/;
   $hash =   Load $yaml;
dd $hash;

##</code><code>##

 0:26 >perl 1186_SoPW.pl
{ Barney => "FRED", Fred => "Wilma", Frederick => "Mary" }

 0:30 >

##</code><code>##

$yaml =~ s/(?<=\n)Fred(?=:)/FRED/g;