You can get rid of the loop in your second example entirely. 
my %subst_for = (
    "walked" => "fed",
    "boy" => "girl",
    "dog" => "Audrey II"
);

$_ = "the boy walked the dog";

my $alt = join '|', map quotemeta, keys %subs;

s/($alt)/$subst_for{$1}/g;

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Makeshifts last the longest.

In reply to Re^2: Chaining string ops by Aristotle
in thread Chaining string ops by traveler

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