Slight modification needed to make this compile. Here it is:

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
}
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Again, thanks for everyone's help. I like chicken.