Because the default is prudently to print using DBL_DIG or LDBL_DIG digits which should be one less than the number of digits necessary to distinguish any possible value. If you feel exceptionally daring, you can override this by configuring perl with something like -Accflags=-DOVR_DBL_DIG=16 or -Accflags=-DOVR_LDBL_DIG=19. I wouldn't recommend it, though.