NB: If the word | substring translations are essentially one-to-one, you can use the technique discussed in haukex's Building Regex Alternations Dynamically article to do a fairly fast search/replace:

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

use Test::More 'no_plan';
use Test::NoWarnings;

my %replace = qw(
    milk white  toast brown  cheese yellow  peas green
    );

my ($rx_search) =
    map  qr{ (?i) (?<! [-\w]) (?: $_) (?! [-\w]) }xms,
    join ' | ',
    map  quotemeta,
    reverse sort
    keys %replace
    ;
print $rx_search, "\n";  # for debug

VECTOR:
for my $ar_vector (
    'no changes in these',
    [ 'milky appease peasoup cheese-toast'
      =>
      'milky appease peasoup cheese-toast'
      ],
    'parts of all these should change',
    [ 'mIlK, some PeAs, cheese and toast.'
      =>
      'white, some green, yellow and brown.'
      ],
    ) {

    if (not ref $ar_vector) {
        note $ar_vector;
        next VECTOR;
        }

    my ($string, $expected) = @$ar_vector;

    (my $replaced = $string) =~ s{ ($rx_search) }{$replace{lc $1}}xmsg
+;
    is $replaced, $expected, "'$string' -> '$expected'";

    }

done_testing;

exit;

__END__
(?msx-i: (?i) (?<! [-\w]) (?: toast | peas | milk | cheese) (?! [-\w])
+ )
# no changes in these
ok 1 - 'milky appease peasoup cheese-toast' -> 'milky appease peasoup 
+cheese-toast'
# parts of all these should change
ok 2 - 'mIlK, some PeAs, cheese and toast.' -> 'white, some green, yel
+low and brown.'
1..2
ok 3 - no warnings
1..3
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