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my $str = "the boy walked the dog";
for ( $str ) {
...
  s/boy/girl/;
  s/dog/Audrey II/;
}

my $str = "the boy walked the dog";
my %subs = ( "walked", "fed", "boy", "girl", "dog", "Audrey II" );
foreach ( keys %subs ) {
  $str =~ s/$_/$subs{$_}/;
}