my %mungers = (
            foo => sub { $_[0] =~ s/foo/bar/g },
                fru => sub { $_[0] =~ s/(fru.*)/uc($1)/e },
              );

if( $phase_of_moon > WAXING_GIBBOUS ) {
  $mungers{foo}->( $string );  # this line changed
} else {
  $mungers{fru}->( $string );  # this line changed
}