$_ = 'taxial';
  my %subst = ( 'taxi' => 'cab', 'cabal' => 'gang' );
  local $" = "|";
  my @keys = map quotemeta, sort { length $b <=> length $a } keys %subst;
  my $regexp = qr(@keys);
  s/($regexp)/$subst{$1}/g;
  print;