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

##</code><code>##

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